WHMCS + WP multisite subdomain auto-provisioning

Hi WPMUDev! We have a WP multisite (subdomain) set up and working, and WHMCS with some products like:
(1) Buy a WP subdomain site (mysite.ourdomain.com);
(2) Buy a domain (somedomain.com) and have it mapped to a WP subdomain site (somedomain.ourdomain.com).

We need help getting WHMCS to auto-populate the subdomain sites in WP when a client completes the purchase. The purchase itself seems fine, we're just not getting a new WP subdomain site created.