You can file as self employed, if you are classified as a professional gambler; but most likely you are not. . It is rare to that a person who has other employment income could classify as a professional gambler.
On the other hand, you would count this as other income, sourced as gambling. But, you would be able to claim expenses of the winnings. So the apportioned cost of accessing the internet to place the wagers, the wager cost, etc.