Edit: Also code for 12 months free in first reply below!
Haven't seen this posted anywhere ever...
I unsuccessfully tried signing up for a free one-year FoundersCard membership via an obscure Amex partnership. I tried various Amex cards that were at my disposal (both U.S. and non-U.S.), but it never accepted any of the numbers. I continued to get spammy e-mails encouraging me to complete registration, but it never worked, and they never replied to my emails asking for help.
The other day, I got a slightly different e-mail about a 6 month membership with a different code and that one worked!
The code doesn't seem personalized, so feel free to give it a shot: viptrialrun6
Enjoy