code: def update_custom_field(custom_field_key, field_name,
        visible_in_preference_center)
        custom_field_key = CGI.escape(custom_field_key)
        options = { :body => {
          :FieldName => field_name,
          :VisibleInPreferenceCenter => visible_in_preference_center }.to_json }
        response = put "customfields/#{custom_field_key}", options
        response.parsed_response
      end
docs: Updates a custom field belonging to this list.
   custom_field_key - String which represents the key for the custom field
   field_name - String representing the name to be given to the field
   visible_in_preference_center - Boolean indicating whether or not the
      field should be visible in the subscriber preference center