It's impossible to trigger it from code at any given time. The event must be user-originated (e.g. by a click listener).
If you, however, want to have a button outside the editor which will trigger this logic, you can use the button delivered by CKEditor 5 in ImageUploadUI. Just do this:
const imageUploadButton = editor.ui.componentFactory.create( 'imageUpload', editor.locale );
imageUploadButton.render();
imageUploadButton.element; // The element that you can insert somewhere in your view.