I have signed up for promotions in two ways. Logging on to the AGR or the regular Amtrak website, since they have now merged. Anyway,I find the desired AGR Promotions advertisement, clicked on it and followed the steps. Sorry, I don't remember the exact clicks. I have also just called the AGR telephone number and had a customer service representative to it.
FWIW, I have signed up for Promotions that I am very unlikely to use. But circumstances change and you never know. For example, I have yet to travel east of of the CONO line, but would like to very much, so I sign up just in case.
When I first joined the AGR program, the CS representative suggested I do that, just in case. Maybe that isn't the right thing to do. If anyone here can advise me differently, I would welcome the information.
At any rate, I hope all works out well for you.