I haven’t used FTX because illegal in USA slash usual worries. So you could deposit BTC, bet on the election, and if you win always get back your BTC+15%? Or not?
The contracts are denominated in USD, and they pay in that currency. But you trade on margin, and the collateral can be in any currency (crypto or fiat). In your example, you get back the BTC plus 15% of what that BTC was worth in USD when you made the trade.
Incidentally, TRUMPFEB is now trading at 0.16 (i.e. implied 16% chance that Trump is president next February). This looks insane to me (and I have bet accordingly). I’d be curious if you or others have further thoughts on what might be going on.
What were (or are now) the best places for US persons interested in betting significant crypto sums?
Side note: the counterparty risk (either from hacks or scams) of FTX seems extremely low right now. The team behind FTX/Alameda has built flawless reputation in the crypto industry over the past few years, and is considered among the strongest technically. They basically went from non-existent to top ~3 of crypto exchanges worldwide in volumes in a matter of ~1 year. I think FTX will continue to grow, as (1) crypto grows as a whole; and (2) FTX innovates and grows more than the competition.
For US residents, www.ftx.us is available (but more restricted).
The short answer is yes: you could bet in BTC because FTX pools collateral across all positions: https://help.ftx.com/hc/en-us/articles/360027946371-Margin-Collateral