I think it's by email.
I had an email from Ancestry a few months ago about 3 free days, & I believe the most recent message on rootschat was from someone who had been told about it by Ancestry. I wasn't notified by Ancestry this time, but thanks to the original poster on here, managed to get six free days, 3 on my pc, then again on OH's laptop.
I certainly got my money's worth

Betty