code: def add_member_to_list(self, username, listname, member_type="USER"):
          
          return self.client.service.addMemberToList(
              listname, username, member_type, self.proxy_id
          )
docs: Add a member to an existing list.
  
          Args:
              username (str): The username of the user to add
              listname (str): The name of the list to add the user to
              member_type (str): Normally, this should be "USER".
                  If you are adding a list as a member of another list,
                  set this to "LIST", instead.