Hi Yma 
Indeed the rich text doesn’t have a behavior where pressing enter could submit a form, unlike regular form inputs, as if you are using a rich text element one would expect users to be able to press enter to edit their text.
You could try to add some event listeners in your page or form to listen for keystrokes.
In this post you can see some of these codes:
(Be aware that this post is talking about blocking key strokes, but some of this thought process could help you find an answer.)