Associate by buyer_id
The easiest way to associate a buyer with a PayPal account is to pass along a
buyer_id when creating a PayPal account.
This will find a buyer with that id and associate the PayPal account with it. If
a buyer with that id could not be found it’s ignored.
Associate by buyer_external_identifier
Alternatively, a PayPal account can be associated with a buyer by using it’s
external_identifier. If a buyer with that external_identifier could not be
found it’s ignored.