Checks to see if a script declaration contains #asyncload or #deferload and inserts * the attribute in the script declaration if it is found. Based on this: https://tinyurl.com/y2obn9os

add_attribute_for_script.php

/**
 * add_attribute_for_script - Checks to see if a script declaration contains #asyncload or #deferload and inserts
 * the attribute in the script declaration if it is found. Based on this: https://tinyurl.com/y2obn9os
 *
 * @param string $url - the URL to check to see if it contains #asyncload or #deferload
 *
 * @return string - the modified string if any change was required.
 */
function add_attribute_for_script( $url )
{
    if ( ( strpos( $url, '#asyncload')===false ) && ( strpos( $url, '#defer')===false ) ) { // No attribute specified.
        return $url;
    }
    else if ( is_admin() ) { // We're in the WP Admin Interface.
        return str_replace( '#asyncload', '', $url );
    }
    else if ( strpos( $url, '#asyncload' )!==false ) { // asyncload was found.
        return str_replace( '#asyncload', '', $url)."' async='async";
    }
    else { // Has to be Defer.
        return str_replace( '#deferload', '', $url)."' defer='defer";
    }
}
add_filter('clean_url', 'add_attribute_for_script', 11, 1);