OK, I found friend's email dated Aug 19th on the SPG FN posting.

<< Got an email from AMEX about my Marriott free night certificate worth 35k. I logged into my account and it seems I click on the "start booking" tab to book my free night.

My closing statement date is September 6, 2018, >>

Seems you need the link from an AMEX email.

EDIT -

asked friend the details

This is where you will find your FN available

<< American Express account, click on amex Spg then clicked on benefit and there was a button to click to book free night. Saw this b4 September 6th closing date (annual fee) >>

Good luck everyone.