first initial last name as username

I have created a registration form page which is working. However when approved, the users username is their first and last name combined. John Doe becomes johndoe.

Previous to a registration page I was adding users manually through the dashboard. Here I can pick whatever username I wanted but standardized it with first name initial and last name. John Doe becomes jdoe as their username.

It seems with the registration form this isnt possible? How can I make this work? I know I have the option to force email as username, but really want first initial lastname for consistency.