⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ Go to your edit profile page in a browser.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Steamcommunity.com/my/edit

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Set one of your showcases to "Favorite Game".

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Right click the page, go to inspect elements, and click the console tab.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Paste the code below into the console and send it by pressing enter.

FavoriteGameShowcaseOnGameChange( this.parentNode, 6, 0, {appid: 753})

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀You can also showcase any game or app on Steam.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Simply replace the numbers after appid in the code above with the appid of
⠀⠀⠀⠀⠀⠀⠀⠀your desired game or app and send the code normally. Here are a few examples:

FavoriteGameShowcaseOnGameChange( this.parentNode, 6, 0, {appid: 765})

FavoriteGameShowcaseOnGameChange( this.parentNode, 6, 0, {appid: 250820})

⠀⠀These are just a few examples. You can use any appid by simply changing the number at the end.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀To find an appid you want to use, you can visit SteamDB and this guide.

⠀⠀⠀⠀⠀⠀⠀⠀⠀You can also set Steam or another app to your "Game Collector" showcase.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Simply use the following codes.

First Game:

SetShowcaseConfig(2, 0, {appid: 753})

Second Game:

SetShowcaseConfig(2, 1, {appid: 753})

Third Game:

SetShowcaseConfig(2, 2, {appid: 753})

Fourth Game:

SetShowcaseConfig(2, 3, {appid: 753})

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀You need to refresh the page for the changes to take effect.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ You can also run Steam and other apps as games using this guide.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Credit for this discovery goes to PsyKitty. Rest in peace.