Does anyone know how to fix my current issue, because this seems to be a very obscure case.

current situation: user is logged in on our app and wants to add a link to their "instagram" profile.
The user presses "add profile" button and has to fill in their username.
Issue: users dont know their username, or misspell them(this causes users to lose interest)

What we want it to be:
already logged in user presses "add" and gets directed to the social media in question to authorize our app which then returns a link to the users profile