Use a variable at the body of the API Call with Oauth2 Custom Token

So, I’ve already made the API Call work with my own personal_token, but I need that every user of the app i’m making gets to config it’s own personal_token, wich is sent at the body of the authentication POST.
Right now, I got this:

When I put my own personal token there, It works… but how do I put a variable in there?
Anyone have already been through something similar, or have any sugestion?

I was abble to solve this problem with some help of the Facebook group.
This is the article that helped me with it: Google Calendar API use case - #7 by mebeingken