I forgot my Apple ID linked with watch and lost original box
What to do when you forget Apple ID you sign in Apple Watch,and original box is lost
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It is almost certain your Apple ID on your Apple Watch is the same as on the paired iPhone. You should be able to find details from your iPhone, go to the Settings app.
Settings > Apple ID ("your name", top entry on the list of settings)
This should open a new screen with all your details. If you have forgotten your Apple ID password you can change it from here on your iPhone and make it something you can remember.
If you forgot your Apple ID password - Apple Support