How to programmatically enter text in comments in FaceBook?

Question

JavaScript

Hello. I'm trying to programmatically enter text into the comment field in FaceBook, but after I try to manually enter something else on the text I've pasted there programmatically, it erases the previous text. I posted the problem on YouTube, check the link and tell me what and how :(
https://youtu.be/NwkbPJ76wmA