Set hostname and username automaticallly

In the installer i don't want to manually create a username, password and hostname. Basically i have a script that that takes the hostname from MAC address, and a systemd service that deletes itself after everything is set up. I want to do this f in such a way that i am not prompted for the above queries.
And where should i put this script? Or do i need to preseed it. If yes then how?