Some non-standard HTML properties are not output to the field HTML by default. This filter allows you to allow or deny specific properties to be added to the field HTML.
The following would apply to all forms.
add_filter( 'gaddon_no_output_field_properties', 'your_function_name' );
This code should be placed in the functions.php file of your active theme.
This filter is located in class-gf-addon.php.