Manually create Apple hidden e-Mail addresses @privaterelay.appleid.com
To use your privaterelay.appleid.com as disposable adresses is not possible at the moment, because it accepts only the developer id who created it as a sender.


  It's important to understand that the email address given to the
  developer (you), in this case:  xxxxxprm23@privaterelay.appleid.com
  
  may only be used from the email domains (with SPF records) or full
  email addresses registered in the Developer Portal.  It is not
  currently possible for you as a developer to hand that email address
  to a third-party and expect to be able to route through the
  privaterelay.appleid.com MTAs.  Unfortunately, it's also not possible
  to register any third-party domain such as mandrillapp.com as one of
  your email source domains.
  
  At this time, mail API services that send email from their own MTAs on
  behalf of their developer customers do not work through
  privaterelay.appleid.com.  The only available workaround currently is
  to setup your own MTA and emit mail from one of your registered
  domains.
  
  We are aware of this issue and are working on a solution that would
  allow developers to use these mail API services.
  
  Stay tuned...


Reference https://forums.developer.apple.com/thread/121256