code: def create_custom_field(field_name, data_type, options=[],
        visible_in_preference_center=true)
        options = { :body => {
          :FieldName => field_name,
          :DataType => data_type,
          :Options => options,
          :VisibleInPreferenceCenter => visible_in_preference_center }.to_json }
        response = post "customfields", options
        response.parsed_response
      end
docs: Creates a new custom field for this list.
   field_name - String representing the name to be given to the field
   data_type - String representing the data type of the field. Valid data
     types are 'Text', 'Number', 'MultiSelectOne', 'MultiSelectMany',
     'Date', 'Country', and 'USState'.
   options - Array of Strings representing the options for the field if it
     is of type 'MultiSelectOne' or 'MultiSelectMany'.
   visible_in_preference_center - Boolean indicating whether or not the
      field should be visible in the subscriber preference center