Coupon is TESTANDCODE
I had already used it before realising it was one or the other, honestly its one of the better TLD’s IMO, too bad Cloudflare doesn’t support it yet.
Edit: I should have mentioned, you have to make a tentative payment of 1 USD, which is reverted.
I can’t speak to long term, but I just registered my first domain through them yesterday and it seemed fine. Better than other small services I’ve used in the past, for sure.