Better version of this guide is on Steam: https://steamcommunity.com/sharedfiles/filedetails/?id=3620288011

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Guide by mv and edited by me

1. Go to your group's Steam page and click Edit Profile (DO THIS IN YOUR WEB BROWSER, NOT THE STEAM DESKTOP APP).
2. Press Ctrl+Shift+I, this'll open up devtools
3. Go to the Console tab
4. Copy and paste this in, you won't see anything change yet (it should return a list of appIDs, though).

var gameList = GetCurrentGameAssociationList();
gameList.push('253530');
document.getElementById('favorite_games').value = gameList.join(',');

5. Click Save, you should see Fortress Forever in the list now.

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Replace 253530 with any other appID to add a different game.