This works for many other AppIDs, but Steam will be used as the main example. The number "753" refers to Steam and can be replaced with any other AppID.

You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start

https://steamdb.info/ is a great website you can use to find AppID's(it's also good for other stuff).

You can also use the following guide to find a bunch of neat hidden AppIDs: Collection of hidden Apps and their compatibility.

This is the string:
Again, the number after AppID(753) can be replaced with other AppID numbers. Not all will work, mainly apps on SteamDB that are listed as "Unkown", you just have to test for yourself.

Scroll to the bottom of the guide to a see a few examples of things you can make as your favourite "game".

You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start.

1. Log into your Steam account in the Google Chrome browser.
2. Go to the Edit Profile page: https://gtm.steamproxy.vip/my/edit
3. Scroll to your Favorite Game showcase.
4. Open the developer console by pressing F12 on your keyboard, or by right-clicking the page and selecting Inspect.
5. Select the Console tab.
6. Paste the following into the console then hit enter:

Refresh the page, making sure to NOT click on Save.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the Favorite Game showcase.

You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start.

1. Log into your Steam account in the Google Chrome browser.
2. Go to the Edit Profile page: https://gtm.steamproxy.vip/my/edit
3. Scroll to your Favorite Game showcase.
4. Open the developer console by pressing CTRL+SHIFT+K.
5. Paste the following into the console then hit enter:

6. Refresh the page, making sure to NOT click on Save.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the Favorite Game showcase.

To use the console in Safari you must first enable the Develop menu.

1. In the toolbar area, go to Safari > Preferences.
2. Click on the Advanced tab.
3. At the bottom of the Advanced tab, click the check-box that is next to the Show Develop menu in menu bar text.
You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start.
1. Log into your Steam account in the Safari browser.
2. Go to the Edit Profile page: https://gtm.steamproxy.vip/my/edit
3. Scroll to the Favorite Game showcase.
4 .Go to the Develop menu in the toolbar and select Show Web Inspector.
5. Select the Console tab.
6. Paste the following into the console then hit enter:
7. Refresh the page, making sure to NOT click on Save.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the Favorite Game showcase.

You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start.

1. Log into your Steam account in the Opera browser.
2. Go to the Edit Profile page: https://gtm.steamproxy.vip/my/edit
3. Scroll to your Favorite Game showcase.
4. Open the developer console by pressing CLTR+SHIFT+I on your keyboard.
5. Select the Console tab.
6. Paste the following into the console then hit enter:
7. Refresh the page, making sure to NOT click on Save.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the Favorite Game showcase.

You must already have a favourite game selected for the string to work. If you do not have one selected simply choose any random game as your favourite game before you start.

1. Log into your Steam account in the Microsoft Edge browser.
2. Go to the Edit Profile page: https://gtm.steamproxy.vip/my/edit
3. Scroll to your Favorite Game showcase.
4. Open the developer by right-clicking the page and selecting Inspect.
5. Select the Console tab.
6. Paste the following into the console then hit enter:
7. Refresh the page, making sure to NOT click on Save.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the Favorite Game showcase.

Update: Many apps(including some of the ones shown below) no longer have an image and instead has an empty/404 image. The name of the app will still show, but no image will accompany it.
I'll let the examples below that presently have no images anymore stay as a legacy.

NB: If your showcase returns with '<None Selected>' after entering the string and refreshing the page it means that the AppID you tried to use doesn't work for the showcase.

Steam - AppID: 753
Team Fortress 2008 - AppID: 943490
Are You Ready for Valve Index? - AppID: 1070910
Ubisoft Test App - AppID: 227140
hackerone_test_app3 - AppID: 1057910
Steam Audio - AppID: 596420
Steam Hardware - AppID: 223300
ValveTestApp102030 - AppID: 102030
ValveTestApp55150 DLC1 - PL - AppID: 55346