I bought a watch online and now it has a activation lock

you can't do anything really, return it

activation lock is not a thing on the ipad it's a theft protection on apples servers

it needs to be removed About Activation Lock on your Apple Watch - Apple Support

if you don't know your appleID password If you have forgotten your Apple ID password – Apple Support (UK)

then you will need the assistance of apple Activation Lock - Support (apple.com)

and or Apple Support

And or Contact - Official Apple Support

and or Genius Bar Reservation and Apple Support Options - Apple