Calling my .NET Core Teams Bot from Angular

I have created a Teams bot in .NET Core from following the sample found here: https://github.com/microsoft/BotBuilder-Samples/tree/master/samples/csharp_dotnetcore/57.teams-conversation-bot

This is working and is running locally with ngrok. I have a controller with a route of api/messages:

[Route("api/messages")]
[ApiController]
public class BotController : ControllerBase
{
    private readonly IBotFrameworkHttpAdapter Adapter;
    private readonly IBot Bot;

    public BotController(IBotFrameworkHttpAdapter adapter, IBot bot)
    {
        Adapter = adapter;
        Bot = bot;
    }

    [HttpPost]
    public async Task PostAsync()
    {
        // Delegate the processing of the HTTP POST to the adapter.
        // The adapter will invoke the bot.
        await Adapter.ProcessAsync(Request, Response, Bot);
    }
}

I now want to call a POST to api/messages from my Angular client using TypeScript to send a proactive message to a specific Teams user.

I did figure out how to set the ConversationParameters in TeamsConversationBot.cs to a specific Teams user by doing the following:

var conversationParameters = new ConversationParameters
{
    IsGroup = false,
    Bot = turnContext.Activity.Recipient,
    Members = new[] { new ChannelAccount("[insert unique Teams user guid here]") },
    TenantId = turnContext.Activity.Conversation.TenantId,
};

but what I'm struggling with is how to build a JSON request that sends the Teams user guid (and maybe a couple other details) to my api/messages route from TypeScript.

How do I go about doing this? What parameters/body do I need to send? I haven't been able to find samples online that show how to do this.

Update below for added clarification

I am building a web chat app using Angular for our customers. What I'm trying to do is send a proactive message to our internal employees, who are using Microsoft Teams, when a customer performs some action via the chat app (initiates a conversation, sends a message, etc.).

I've built a Teams bot using .NET Core using this sample: https://kutt.it/ZCftjJ. Modifiying that sample, I can hardcode my Teams user ID and the proactive message is showing up successfully in Teams:

var proactiveMessage = MessageFactory.Text($"This is a proactive message.");
var conversationParameters = new ConversationParameters
{
    IsGroup = false,
    Bot = turnContext.Activity.Recipient,
    Members = new[] { new ChannelAccount("insert Teams ID here") },
    TenantId = turnContext.Activity.Conversation.TenantId,
};

await ((BotFrameworkAdapter)turnContext.Adapter).CreateConversationAsync(teamsChannelId, serviceUrl, credentials, conversationParameters,
    async (t1, c1) =>
    {
        conversationReference = t1.Activity.GetConversationReference();
        await ((BotFrameworkAdapter)turnContext.Adapter).ContinueConversationAsync(_appId, conversationReference,
            async (t2, c2) =>
            {
                await t2.SendActivityAsync(proactiveMessage, c2);
            },
            cancellationToken);
    },
cancellationToken);

What I'm struggling with is:

1. How to configure my Angular app to notify my bot of a new proactive message I want to send.
2. How to configure the bot to accept some custom parameters (Teams user ID, message).