First of all, you will have to open your account settings on google chrome or mozilla firefox.

Then, when you selected favorite game as your showcase, right click it and hit inspect element.

When you are into inspect element, look for the following code:
When you found it, right click it and hit Edit as HTML

A lot of code will then pop up. Do not worry about this code, the only thing that is important for us is this line of code:
In value="", you need to insert the app id. You can find those on this website[steamdb.info].

But for example, Steam's app ID is 753. So your code should look like this:

Then, just click anywhere else in the developer console to lock in the changes.

Then, click save and you're done!