I've done this before but in the reverse (use a Cdn card on US website.) Obviously, you can't sign in to your account. You must use guest mode. IIRC, I entered my correct street address, but used a fake state and zip code. Can't remember what I used for the city. I believe it worked for either MC or Visa, but not both. You'll have to experiment. But the transaction went through. Once I had my PNR, I just added my UA number and it appeared in my account. No issues.