docs: def add_customers(self, service_desk_id, list_of_usernames):
          """
          Adds one or more existing customers to the given service desk.
          If you need to create a customer, see Create customer method.
  
          Administer project permission is required, or agents if public signups
          and invites are enabled for the Service Desk project.
  
          :param service_desk_id: str
          :param list_of_usernames: list
          :return: the customers added to the service desk
          """
          url = 'rest/servicedeskapi/servicedesk/{}/customer'.format(service_desk_id)
          data = {'usernames': list_of_usernames}
  
          return self.post(url, headers=self.experimental_headers, data=data)