You can use the WinAPI PostMessageW() function to do the task.
You can use #include <WinUser32.mqh> to import the WinAPI function descriptions.
Then you can use these commands to perform the task you need.
(For trendline #define MT4_WMCMD_TRENDLINE 33257)
Here you can find the commands
#define MT4_WMCMD_AUTOSCROLL 33017
#define MT4_WMCMD_BARS 33018
#define MT4_WMCMD_CANDLES 33019
/* 33020 unknown */
#define MT4_WMCMD_GRID 33021
#define MT4_WMCMD_LINE 33022
#define MT4_WMCMD_SHIFT 33023
#define MT4_WMCMD_VOLUMES 33024
#define MT4_WMCMD_ZOOM_IN 33025
#define MT4_WMCMD_ZOOM_OUT 33026
/* 33027-33047 unknown */
#define MT4_WMCMD_EXPERT_INPUTS 33048 /* Open expert Inputs dialog */
/* 33049 unknown */
#define MT4_WMCMD_REMOVE_EXPERT 33050 /* Remove expert advisor from chart */
/* 33051-33053 unknown */
#define MT4_WMCMD_SAVE_AS_PICTURE 33054
#define MT4_WMCMD_3_MONTHS 33057
#define MT4_WMCMD_ALL_HISTORY 33058
#define MT4_WMCMD_LAST_MONTH 33063
#define MT4_WMCMD_SAVE_Statement 33064
/* 33055-33133 unknown */
#define MT4_WMCMD_PERIOD_D1 33134
/* 33135 unknown */
#define MT4_WMCMD_PERIOD_H4 33136
#define MT4_WMCMD_PERIOD_M1 33137
#define MT4_WMCMD_PERIOD_M5 33138
#define MT4_WMCMD_PERIOD_M15 33139
#define MT4_WMCMD_PERIOD_M30 33140
#define MT4_WMCMD_PERIOD_W1 33141
/* 33142-33156 unknown */
#define MT4_WMCMD_PROPERTIES 33157 /* Chart Properties (F8) */
/* 33158-33176 apparently unused */
#define MT4_WMCMD_PERIOD_SEPARATE 33177 /* Show/hide period separators */
/* 33178-33196 unknown */
#define MT4_WMCMD_MOVE_RIGHT 33197 /* Move right (right/down arrow) */
#define MT4_WMCMD_MOVE_LEFT 33198 /* Move left (left/up arrow) */
/* 33199-33219 unknown */
#define MT4_WMCMD_SAVE_TEMPLATE 33220 /* Save template... */
/* 33221-33229 unknown */
#define MT4_WMCMD_33230 33230 /* Arrow Down */
#define MT4_WMCMD_33231 33231 /* Arrow Up */
#define MT4_WMCMD_33232 33232 /* icons-ticks */
#define MT4_WMCMD_MOUSE_CROSS 33233 /* crosshair */
#define MT4_WMCMD_MOUSE_CURSOR 33234 /* the cursor */
#define MT4_WMCMD_33235 33235 /* Box-cyclical Line */
#define MT4_WMCMD_33236 33236 /* equidistant channel */
#define MT4_WMCMD_33237 33237 /* Fibonacci lines */
#define MT4_WMCMD_33238 33238 /* Box-Fibonacci-arc */
#define MT4_WMCMD_33239 33239 /* Box-Fibonacci-fan */
#define MT4_WMCMD_33240 33240 /* Box Channels, Fibonacci */
#define MT4_WMCMD_33241 33241 /* Box-Gann-grid Hanna */
#define MT4_WMCMD_33242 33242 /* Box-Gann-line Hanna */
#define MT4_WMCMD_33243 33243 /* Box-Gann-fan Hanna */
#define MT4_WMCMD_HLINE 33244 /* horizontal line */
/* 33245 unknown */
#define MT4_WMCMD_33246 33246 /* Box Andrews pitchfork */
#define MT4_WMCMD_33247 33247 /* Box-shape-rectangle */
#define MT4_WMCMD_33248 33248 /* Box-channels-linear regression */
/* 33249-33250 apparently unused */
#define MT4_WMCMD_33251 33251 /* Box-icons-stop */
/* 33252 unknown */
#define MT4_WMCMD_33253 33253 /* text */
#define MT4_WMCMD_33254 33254 /* Box-icons-bad */
#define MT4_WMCMD_33255 33255 /* Box-icons-well */
#define MT4_WMCMD_33256 33256 /* Box-Fibonacci-time-zone */
#define MT4_WMCMD_TRENDLINE 33257 /* trend line */
#define MT4_WMCMD_33258 33258 /* Box-Line-trend line for the corner */
#define MT4_WMCMD_33259 33259 /* Box-figure-triangle */
#define MT4_WMCMD_VLINE 33260 /* Vertical Line */
#define MT4_WMCMD_LABEL 33261 /* text label */
/* 33262-33323 unknown */
#define MT4_WMCMD_UPDATE_DATA 33324 /* This doesn't cause experts start() to run */
/* 33325-33333 unknown */
#define MT4_WMCMD_PERIOD_MN1 33334 /* timeframe MN */
/* 33335-34299 unknown */
#define MT4_WMCMD_34300 34300 /* 34300-34699 custom indicators in alphabetical order */
#define MT4_WMCMD_34700 34700 /* Indicators-moving average of oscillator */
#define MT4_WMCMD_34701 34701 /* Indicators-accelerator oscillator */
#define MT4_WMCMD_34702 34702 /* Indicators-accumulation/distribution */
#define MT4_WMCMD_34703 34703 /* Indicators-alligator */
#define MT4_WMCMD_34704 34704 /* indicators-average directional movement index */
#define MT4_WMCMD_34705 34705 /* indicators-average true range */
#define MT4_WMCMD_34706 34706 /* Indicators-awesome oscillator */
#define MT4_WMCMD_34707 34707 /* Indicators-bears power */
#define MT4_WMCMD_34708 34708 /* indicator-bollinger bands */
#define MT4_WMCMD_34709 34709 /* Indicators-bulls power */
#define MT4_WMCMD_34710 34710 /* Indicators-commodity channel index */
#define MT4_WMCMD_34711 34711 /* Indicators-demarker */
#define MT4_WMCMD_34712 34712 /* Indicators-envelopes */
#define MT4_WMCMD_34713 34713 /* Indicators-force index */
#define MT4_WMCMD_34714 34714 /* Indicators-fractals */
#define MT4_WMCMD_34715 34715 /* Indicators-gator oscillator */
#define MT4_WMCMD_34716 34716 /* Indicators-ichimoku kinko hyo */
#define MT4_WMCMD_34717 34717 /* Indicators-macd */
#define MT4_WMCMD_34718 34718 /* indicators-market facilitation index */
#define MT4_WMCMD_34719 34719 /* Indicators-momentum */
#define MT4_WMCMD_34720 34720 /* indicators-money flow index */
#define MT4_WMCMD_34721 34721 /* indicators-moving averages */
#define MT4_WMCMD_34722 34722 /* Indicators-on balance volume */
#define MT4_WMCMD_34723 34723 /* Indicators-parabolic sar */
#define MT4_WMCMD_34724 34724 /* indicators-relative strength index */
#define MT4_WMCMD_34725 34725 /* indicators-relative vigor index */
#define MT4_WMCMD_34726 34726 /* Indicators-standard deviation */
#define MT4_WMCMD_34727 34727 /* Indicators-stochastic oscillator */
#define MT4_WMCMD_34728 34728 /* Indicators-volumes */
#define MT4_WMCMD_34729 34729 /* Indicators-william's percent range */
/* 34730-34799 unknown */
#define MT4_WMCMD_LOAD_TEMPLATE 34800 /* 34800-34899 load template in alphabetical order */
#define MT4_WMCMD_REMOVE_TEMPLATE 34900 /* 34900-34998 Remove Template in alph. order (conf. dialog) */
#define MT4_WMCMD_KILL_TEMPLATES 34999 /* remove *.tpl (confirmation dialog) */
/* 35000-35399 unknown */
#define MT4_WMCMD_PERIOD_H1 35400 /* timeframe H1 */
#define MT4_WMCMD_OBJECTS_LIST 35402 /* open objects list window */
#define MT4_WMCMD_ORDER_BUYLIMIT 35454 /* order buy limit */
#define MT4_WMCMD_ORDER_BUYSTOP 35455 /* order buy stop */
#define MT4_WMCMD_ORDER_SELLLIMIT 35456 /* order sell limit */
#define MT4_WMCMD_ORDER_SELLSTOP 35457 /* order sell stop */
#define MT4_WMCMD_ORDER_MARKET 35458 /* new order (F9) */
#define MT4_WMCMD_35464 35464 /* right price tag */
#define MT4_WMCMD_35511 35511 /* Graphic template-download template */
/* 35512-37011 unknown */
/* 37012-40000 not inspected */
#define MT4_WMCMD_57602 57602 /* close the current chart */
#define MT4_WMCMD_57603 57603 /* Save as (ascii/csv) */
#define MT4_WMCMD_57604 57604 /* Save as (ascii/csv) - apparently same */
#define MT4_WMCMD_PRINT_DIALOG 57607 /* Open Print... Dialog */
#define MT4_WMCMD_PRINT_DIRECT 57608 /* Print immediately */
#define MT4_WMCMD_PRINT_PREVIEW 57609 /* Open Print Preview dialog */
/* 57610-59675 unknown (re-checked up to 58011) */
/* 59675-... not inspected */