Once the trademark has registered or the application has been filed, the owner of the mark has to file an application with Amazon Brand Registry. Amazon will then send a verification code to the contact listed with the trademark office where the trademark was registered or filed. The Verification code is then sent to the owner to activate his account in the program.
For the US, if you are already registered on Seller Central as a seller, you can request the verification code once the application has been filed.