It occurs to me that if you don’t want to search from choice you can do the following:
If you have a screen with user profiles, when you know who you want to chat with, enter the profile detail and there configure a form button to request chat directed to the message sheet, as they told you above.
Being inside their profile, you can add their email (receiver), your email (active connected to the users table) and then a unique ID, etc.
It remains to be seen if you need any type of acceptance by the other user. It could also be configured.
I would do as Lisa said, but point 5.
The screen could be the profile of the user you want to chat with.
In point 8, being inside the recipient’s profile, you already have the data, you only have to add all the columns you want to the message sheet. And if you have configured the login with that profile sheet, the sender email can also get it.
I hope that helps.