You just email them at
support@pokerstars.com and give them the tournament number, the buy in, the time and ask them to pre-register you. If you don't get it all right, or don't include all that data, they might email you to verify that you're talking about the same tournament (and if you don't reply to the verification they won't pre-register you).
Also, once registration closes, you can't get out. If you don't make it home in time to play, you're going to get blinded off.