Auto create subsite at user registration

Hi there! I use New Blog Templates to create a subsite at a user's request. Currently, my user gets created during a WooCommerce purchase. He or she is directed via the purchase confirmation email to a custom /wp-signup.php to create the subsite. I'd like to automate the creation of the subsite at user registration, to avoid having the user create their own. The subsite would be username.example.com. I know this would involve the two hooks, user_register and wpmu_create_blog(), but I don't know how to connect them. Can anyone help? TIA.