How to pair a Apple Watch to your phone someone gave you but forgot the Apple ID
How can I pair a Apple Watch to my phone someone gave me and they don’t remember the Apple ID
iPhone 12 Pro Max, iOS 14
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The person that forgot their apple id needs to do this;
Request an Online Activation Lock Removal here.
Or this;
If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.
Otherwise the watch will be of no use to you.