Add a word to the first // wp_get_attachment_url // as the subdomain name

We use this code to display the link …

$  mp3Link = wp_get_attachment_url($  mp3_file_id); <a class="w3-black" href="'.$  mp3Link.'" rel="nofollow" >mp3</a> 

It looks like this now :

I want it to be like this :

I mean, I just want to do this by editing the above code