If you were trying to follow the other user you would need to
a) make sure you’re using the access token belonging to the account that is performing the follow action (your app’s account probably?)
b) ask the user to provide their screen name (or if you’re authenticating the user, obtain it from the account/verify_credentials methods)
c) issue this friendships/create request setting the screen_name or user ID to the user that you’re wanting to follow.