Or you can give united.com bogus credit card data that won't go through - it'll be booked, you get 24 hours to call in correct CC data.

Or you can fill in the booking with the exception of leaving the CC fields blank, then just before hitting the purchase button you can turn off Javascripting in your browser. This will disable all field checking, and submit your reservation to UA with no CC data. Same result as above - 24 hours to call in and fix, or your reservation disappears.

These have the advantage of being applicable to any fare,and reserving a seat assignment with your booking. AFAICT "Book and Hold" doesn't hold your a seat assignment.