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.
Why would you need to transfer them to cloudflare? My .dev domain is $15/yr from gandi.com, and I just change the nameserver to my Cloudflare.