score:4
In the absence of other feedback I feel fairly confident converting my comment to an answer.
If you are taking part in this event for fun, then a Temporary Resident Visa (Visitor Visa) is entirely appropriate for the activity. Plenty of people already come to Canada to visit casinos, or a horserace, and to gamble while they are there. They do so on a Temporary Resident Visa.
If you are a US citizen then you do not need an actual visa to enter Canada for purposes covered by the TRV. You might be asked for your purpose in entering Canada, and "tourism" will do just fine. (I would go with "pleasure" rather than "tourism", but it won't make any difference).
If you make your living playing poker then that's an entirely different matter and you will need some kind of visa that permits work.
Upvote:1
A US citizen physically within Canada who gambles online, at a website based outside the United States, is not violating any US state or federal law.