Enter text into Facebook 'Make post' text input using JS only

Looking for a way to enter text into Facebook's desktop web 'Make post' text input using JS only (and not using Facebook APIs). Picture of exact part of Facebook I'm referring to below - it's found at the top of your News Feed.

Note: I only want to get to the stage of entering text ready to be posted - not actually executing on the post. But if 'Post' button is clicked, post should work, using entered text (this is important).

Some things I've found out so far:

• there is no DOM way to actually trigger keyevents leaving the browser-sandbox - so it feels like this method would be a non-starter
• altering the 'value' of the text field does not work (at least not in the straight-forward way I've been trying), because when I click 'Post', it seems to only take typed in text, instead of text that was assigned in the value field of the element. Because of this, I suspect there's something more complicated going on behind the scenes on Facebook
• to 'open up' the 'Make post' text input - in other words get it from the state in the first screenshot to the state in the second - I'm registering a click event in the middle of the text field. My theory is that I need to 'open up' the input in order to make it feasible for text to be entered into the text input in some way