Regardless of the tax form you receive, as an individual you must report gambling winnings (including sports bets) on your Form 1040, Schedule 1, Line 8, under Other Income. Remember: This is any and all winnings. You can only deduct losses if you itemize your taxes. The same is true of up-front money that you stake.
Yes, winnings from Stake.us may be considered taxable income. For this reason, you need to report any winnings so that you can pay taxes on winnings at Stake.us.