You can participate using any currency, as long as the bet amount is equivalent to the required USD amount.
You can use Bonus buys, but they make it more challenging to reach the target multiplier. The winning multiplier is based on the total bet amount, not your base bet.
Rewards are credited in the next 48 hours after you completed a challange.
Your bet only count if the challenge is active. Your bet is not eligible if it was made before the challenge was live or after the challenge has already been completed by someone else.
Where can I find my Bet ID?
After you hit the target multiplier go to the casino "Bet History" page.
Search your winning bet, open it and copy the bet id. (Help on the picture)