Social network. Simple pet project [closed]

Trying to create a simple social network to shape my skills a little bit.
So I encountered a problem that I can not solve myself.
So I want to add a user to the friend list. So a user can send a request to another user.
So the client app on React can send an API request to the server. And then the server needs to get reached another client app to show the icon that the new friend request received. How can I achieve that?