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There is a well documentated tutorial on how to create add ons. It has helped me a lot. You can find it here.

https://xenforo.com/community/threads/creating-an-addon.5416/

There are also a few guides on how to include logging into XF from an other application. Please see this link for one of them. I hope that they will be of help to you.

https://xenforo.com/community/threads/login-logout-on-external-page.95362/