Getting Postmaster to send Member Account Validation Mails

I’ve set up Postmaster to send the Member Account Validation Mail replacing the default function and using my own html mail template via the “Send System Email” hook. This fires when an admin validates a member from the “Pending” group.

All works fine except the Username is actually the Username of the Admin who validates the New Member. How can I get the Username of the validated member in there instead?

Additionally is it possible to send a password in this manner?