Install WordPress Multi-Site on subdomain

Hey there,

I am wondering how I can install WordPress Multi-Site on a subdomain in a way that it will still permit my users to register subsites as a subdomain and not a subdirectory.

For example:
http://www.mydomain.com(primary site)
app.mydomain.com(wordpress multisite installed)

Users will signup for a site using the primary site domain but then login to the wordpress multisite within the app.mydomain.com directory.

How can I do this?

Thank you,
Landon