Post Loop Widget

Hello

For the Post Loop Widget I would like to add the code equivalent of this into the Additional field in the Build Posts Query box;

$featured = array (
    'post_type' 	 => 'custom_post_a',
    'meta_key'		=> 'select_yes_no',
    'meta_value'	=> 'select_yes'
);

$featured_posts = new WP_Query( $featured );

I can select ‘custom_post_a’ using the Post Type drop down, but how do I add the meta_key and meta_value criteria to the Additional field I have tried the following to no avail.

$featured = new WP_Query( array ( 'meta_key' => 'select_yes_no', 'meta_value' => 'select_yes' ) );