Unity user authorization API with return value,Unity, app needs user's authorization which returns an api key

What I need the app to do is to send a user over to a link with some parameters, like the ID of my app, what I want the response from the API to be. I achieved this with Application.Url(url);. The user then can authorize the apps permission to access this users account and stuff, so he can post on forum or chat with his username, like he would do in-game. BUT! After the authorization from the user, the api returns a user specific api key that I need the app to further use. But I don't know how to do this. Using UnityWebRequest won't let the user click on the authorization button but Application.Url(); won't return any value from the web page.
Does anyone have a way to do this ? I'm sure there is a way, but I can't think of anything rn.