How to add Amazon Pay to an Existing Seller Central Account?

I have a client that I am attempting to setup an Amazon Pay account for.

• Client has an existing Amazon Seller Central (Europe) account and wants to use Amazon Pay (as a business)
• There is no mention of Amazon Pay or Integration Central under their Seller Central account (when logged in)
• On trying to “sign-in” via Amazon Pay, the destination is the existing Seller Central
• On trying to “register your business” with the same client email address (as the existing Seller Central account), prompted to “Create Account” or “Sign In”
• When choosing the “Sign In” option, i get a broken error page saying the email address has already been used (see screenshot)

Can Amazon Pay accounts/merchants be added under existing Amazon Seller Central accounts - if so how?

OR

Does Amazon Pay have to be created under a new email address and kept seperate to any other, existing Amazon Seller Central accounts?

ADDITIONALLY

If the business owner has two different websites and/or businesses that he wishes to use Amazon Pay on, do completely seperate Amazon Pay accounts need to be created

OR

Can an individual Amazon Pay account hold multiple merchant accounts (I’m guessing the answer is yes, but the question is equally, how)?