// Change 'default' if your form has a different ID
add_filter( 'propertyhive_search_form_fields_default', 'edit_default_property_search_form_fields', 101 );
function edit_default_property_search_form_fields($fields)
{
    $fields['marketing_flag'] = array(
        'type' => 'hidden',
        'value' => 123 // Where 123 is the ID of your marketing flag
    );

    return $fields;
}