How do Canadians retrieve tourney money withheld by IRS?

[RaiseAndPray]

Hey guys. I apologize if this topic is covered elsewhere, however I searched extensively and can't find anything.

I am Canadian. I recently placed 2nd in a Canterbury tourney and had $5219.10 withheld by the casino to give to the IRS. I was wondering if anyone has any experience in the process of getting that back? I know of a few Canadians who have retrieved this withheld money (plus a small amount of interest in some cases!), but I can't find good information online about how to do it or where to start. Could anyone please lead me in the right direction?

I know there are services which do this for you for a fee, however I was hoping to do it on my own. Any help is much appreciated!