If this is to be a paying job, it belongs in the employment forum, where that’s allowed.
Simplest way would likely be using a .htaccess redirect.
This will *not* pick up non “www” addressing, but it can be adapted to do so. Test this before deploying on anything important.

Code:

<IfModule mod_rewrite.c>
    RewriteCond %{HTTP_HOST} ^www\. [NC]
    RewriteRule (.*) https://subdomain.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
</IfModule>