code: def validate_tpa_user_id(self, value):
          
          enterprise_customer = self.context.get('enterprise_customer')
  
          try:
              tpa_client = ThirdPartyAuthApiClient()
              username = tpa_client.get_username_from_remote_id(
                  enterprise_customer.identity_provider, value
              )
              user = User.objects.get(username=username)
              return models.EnterpriseCustomerUser.objects.get(
                  user_id=user.id,
                  enterprise_customer=enterprise_customer
              )
          except (models.EnterpriseCustomerUser.DoesNotExist, User.DoesNotExist):
              pass
  
          return None
docs: Validates the tpa_user_id, if is given, to see if there is an existing EnterpriseCustomerUser for it.
  
          It first uses the third party auth api to find the associated username to do the lookup.