A medal
We need to view the whole conversation in context.
Denise was able to post, which means she knows the answer.
And no, no “Soviets" helped me get into your little secret hiding place here. Did you really think I wouldn’t remember when you gave Megan your father’s medal, then blew up at her because she accidentally misplaced it? Megan was so terrified, it took months for me to get her to tell me what happened.
She implies that she was able to get into the website because she remembers when he gave Megan a medal, which indicates that is the answer.
With regard to the larger question of what the precise passphrase is, I doubt there is one. Certainly letting users post would be of no benefit (and would harm the website), which means that any answer would have to do something different (and thus be inconsistent with the conceit of the site). It is probably there merely for consistency with the in-universe story.
Various passphrases, such as "grampa's medal," "your medal,"
"pappy's medal," "medal," "a medal," "the medal," "grandpa's medal" etc. produce no result.
