Custom welcome page is a blank slate, it can be anything.
If you want a UI or behavior similar to default community launcher (e.g. pinned apps) you can do
cp /usr/share/httpd/noindex/nethserver /usr/share/httpd/noindex/mylauncher
and start from there.
In default community launcher pinned apps are retrieved by some jQuery code that execute a PHP script that invokes a Cockpit API.