First we see the text on the page: "You must specify a nick". After quick look into source code of the page we understand that our URL must contain GET-parameter 'nick' with random value.
Then server sends us some leet-modified string like
51xty tw0 plu5 0n3
and expecting from us solution of this expression in the same format.
Experimentally found that there is only 4 leet-modified characters: '1' == 'i', '3' == 'e', '5' == 's', '0' == 'o'.