Plugin Question: How do I set the value of post_textbox in a script?

I would like to set the value of the element id=“post_textbox” from a plugin script.

This code did not work.
The wording is set on the screen, but the submit button is not enabled.
Perhaps the ReactNode is not recognizing the value update.

window.document.getElementById(‘post_textbox’).value += ‘test’

Is there any ideas?
Best regards.