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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)