how do I unlock the activation lock on an apple watch if I forgot my Apple ID?

Is it your Apple ID?

If you forgot your Apple ID - Apple Support

You can 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.