trying to add extra field using hooks

I’m not familiar with the subscription plugin you’re using. But as you’re attaching a custom function to a action hook, you should most likely echo or print the html you want to be displayed at that particular point. Like this,

function custom_domain( $post_id ) {
  // if this is saved as post_meta then get it with get_post_meta( $post_id, 'some_meta_key', true );
  $domain = 'someurl.com'; 
  printf(
    '<input type="text" name="Domain" value="%s">',
    esc_url( $domain )
  );
}