Make your moment easy to recognize.
Use the stage and approximate set time
“Near the left speakers during the final half hour” narrows the search more than a clothing description. Include the city and year.
Tell the small story
A bracelet exchange, a shared laugh, or a conversation on a break can be a useful detail. Don’t post anyone’s handle or a stranger’s photo.
Give the other person a choice
A post is an invitation, not a request for the internet to track someone down. Send one thoughtful reply and respect a lack of response.