How do I do this???

Hi,

I have a App "myApp" and contains users. than there is an external App "ExtApp". myApp & ExtApp have a partnership. So when one user of myApp want to join ExtApp the user should automatically become a user to the external app. So get an account there. I imagine bulding a sdk or something like this for that external app.

How do I build something like this? Can you give me hints?

Here is a picture how I want to do this.
https://imgur.com/a/OI68Bng