[MP2]

Hostnames don't update regularly in the SA:MP browser, so unless someone repeatedly presses the refresh button it's not going to do what you want. It updates about every 10 seconds. Here's the basics of what you need though:

pawn Code:

forward hostname_change();
public hostname_change()
{
    new hn[64] =  "hostname NEW_HOSTNAME_GOES_HERE";
    SendRconCommand(hn);
}