That URL resolves to this one:
https://enroll.amexnetwork.com/US/aa1/

This is a peculiar offer because it is hosted on
amexnetwork.com rather than americanexpress.com. This website is for cards issued by Bank of America, Barclay's, Citi, GE, HSBC, etc in addition to cards issued by American Express. However, the offer is for Membership Rewards which is only available on cards issued by American Express itself. I don't see what they gain by publishing this on amexnetwork rather than their own site.