Add Defer & Async Attributes to a single WordPress Script

Add this to the functions.php

function add_async_attribute($tag, $handle) {
    if ( 'my-js-handle' !== $handle )
        return $tag;
    return str_replace( ' src', ' async="async" src', $tag );
}
add_filter('script_loader_tag', 'add_async_attribute', 10, 2);

//* If you want to use defer just replace async="async" with defer="defer".
//* You will need to change the handle which will be the first parameter of the enqueue method.

Full article: : https://www.evemilano.com/2017/10/aggiungere-defer-async/