code: def add_field_value(self, field, value):
          
          super(Issue, self).update(fields={"update": {field: [{"add": value}]}})
docs: Add a value to a field that supports multiple values, without resetting the existing values.
  
          This should work with: labels, multiple checkbox lists, multiple select
  
          :param field: The field name
          :param value: The field's value
  
          :type field: str