The United Explorer credit card has a $50 rebate on your 1st and 2nd prepaid hotel stays purchased directly through United Hotels.

Thus far, I've found zero hotels available for prepaid stays. Every single one I've looked up has only a "reserve" button to pay later, which wouldn't qualify. The search options are incredibly unhelpful, as they include, under "Payment type", a check-box for "Reserve now, pay later", which is exactly what you don't want, but no way to choose prepaid, which is what you do want.

How can you ever find hotels that you can use the rebate with?