Add data attribute tag to menu link

Hello

I'm trying to get the contact link in my menu to open up a pop window but it requires data-rel="lightcase:myCollection" to be added to the A link.

I've tried to follow this to get it to add something similar...

https://codex.wordpress.org/Plugin_API/Filter_Reference/nav_menu_link_attributes

I've set up a hook to appear in wp-footer with the below script but can't seem to get it to work...

<script>
function add_specific_menu_location_atts( $atts, $item, $args ) {
// check if the item is in the primary menu
if( $args->theme_location == 'primary' ) {
// add the desired attributes:
$atts['data-rel'] = 'lightcase:myCollection';
}
return $atts;
}
add_filter( 'nav_menu_link_attributes', 'add_specific_menu_location_atts', 45 );
</script>

Any ideas what I'm doing wrong?

Link is... http://185.20.51.60/~pjdservicesco/

Thanks
Dave