I’m not really sure how that would be done. The leaderboard is synced directly from the Strava information, which does not show forum names.
You could check with hozn, who runs the leaderboard, on whether there is a way to do this. Or whether he would be willing to put up a spreadsheet with forum names linked with Strava names somewhere on his site, which would probably be the more practical option. But in the absence of that, the easiest way to contact someone on the leaderboard is to click on the name, which takes you to their Strava page, then write the message as a comment on a recent ride. You can even give your forum name, and ask that they contact you on the forum.
.
