How would you prereigister for a GP in another country?
You should check with the Tournament/Event Organizer. They will usually have their own webpage dedicated to the information about the Grand Prix they are running, and it usually details the means with which you can preregister (usually online with credit card or other internet means).
Similarly, the DailyMTG section also has an Event Coverage section with the Grand Prix information pages, but this has not been updated for 2014. You can often still find the appropriate Grand Prix page with a bit of Google. (For example, searching "GP Prague 2014" off of DailyMTG will turn up this page for Event Information for Grand Prix Prague this upcoming weekend.)