how to auto login users in my website when they visit from trusted web sites

I have a web app ( App1) which uses Oauth2.0 compliant server for authenticating the users, There is another web app ( App2) which uses different login system but is a trusted source,

so, A user login to App2 and then click a link which takes them to App1 and will be asked to login and if the attempt is successful, all their subsequent visits to App1 from App2 at any later date should auto login the user.

This is just to facilitate to users of App2 to gain access to App1 seamlessly.

can someone please suggest me some ideas?