I've found a run that would take gold on here and would like to add it on behalf of the person.
If you're the moderator of a game, then you can do this, you just edit the "Player 1" input field and change it to the name of that person. You'd then select that person's SRC account or set them as an unregistered player.
If you aren't a moderator, then you'd have to ask one of that game's moderator(s) to do the above for you. You could ask them by submitting this player's run under your name and writting in the description of who you want the player changed to, or you could just message the moderator(s) about it and talk to them.
If they already have a SRC account, then I'd recommend that you get this person's permission before you submit their run, as they might not want it on the site. But if they're a popular YouTuber or something, then I wouldn't worry about it too much.
Also here's a tip: if an unregistered player's name is already taken by a SRC account on the site, then you can add two square brackets in front of their name "[]". This turns them back into an unregistered player. You can also add a country code between those brackets which displays a flag next to their name, this allows you to show where that player is from.
You can find a country code by finding the image of the flag you want on the site, and then by viewing that image's URL. The code is on the end of that URL.