HELP. HOW CAN I REGISTER MY ANDROID APP TO INTEGRATE WITH FORERUNNER?

That is a developer question for the Developer forum:  Developer 

Check the develper page for how to get data from Garmin: https://developer.garmin.com/

The developer program is currently paused so you can't get access to the APIs. Read more here: https://www.themomentum.ai/blog/garmin-developer-program-closed-roadmap

There are some open source projects that you can use to get the data from Garmin, but nothing official. I'm using garth-ng in my python scripts to fetch some data: https://github.com/cyberfossa/garth-ng but running that on an Android phone is not easy. Running it on a server or your computer will work better. Then it can fetch data and upload it so your app can access it.

Thks for all. So, I will try it on github. In my case, I have got a Android App and need to pair watch data.

You can develop a watch app or data field and then let your phone app be a companion app. You can't read data directly from the watch. 

wow...I did not know...Thanks again. I need to do the step 1 like you said