New site has no place for this, as far as I can see.

It asks for Offer Code, but when I entered e-cert number, I got a message
united.com was unable to retrieve the rule(s) pertaining to the program at this time. Please contact united.com Support at 1-800-396-1751 for assistance in redeeming your eCertificate offer.
I'm using 20% off e-cert.