code: def add_field_to_work_item_type(self, field, process_id, wit_ref_name):
          
          route_values = {}
          if process_id is not None:
              route_values['processId'] = self._serialize.url('process_id', process_id, 'str')
          if wit_ref_name is not None:
              route_values['witRefName'] = self._serialize.url('wit_ref_name', wit_ref_name, 'str')
          content = self._serialize.body(field, 'AddProcessWorkItemTypeFieldRequest')
          response = self._send(http_method='POST',
                                location_id='bc0ad8dc-e3f3-46b0-b06c-5bf861793196',
                                version='5.0-preview.2',
                                route_values=route_values,
                                content=content)
          return self._deserialize('ProcessWorkItemTypeField', response)
docs: AddFieldToWorkItemType.
          [Preview API] Adds a field to a work item type.
          :param :class:`<AddProcessWorkItemTypeFieldRequest> <azure.devops.v5_0.work_item_tracking_process.models.AddProcessWorkItemTypeFieldRequest>` field:
          :param str process_id: The ID of the process.
          :param str wit_ref_name: The reference name of the work item type.
          :rtype: :class:`<ProcessWorkItemTypeField> <azure.devops.v5_0.work_item_tracking_process.models.ProcessWorkItemTypeField>`