How to add "my favorite" button?

I have a table of all the projects listed. I need to add a button for logged-in users to be able to add one of the projects to his/her favorite project list automatically.

So basically, this is a “many to many” connection between the project table and the user table. I tried to add a action button to do this. But I don’t know how to show the user that this project has been added. Or prevent the user from adding it again.

You could reverse it:
Projects 1:M Users

Users project

Thank you. How come I could not think about it.