Getting a Preview Icon of a Prefab.

Hello! First I want to start by saying I'm a complete noob when it comes to modifying the Unity editor itself. So if someone could help me out it would be super appreciated. All I want to do is simply have a custom icon that replaces the default prefab icon in Unity. I know it's possible because other assets that I have gotten from the store are doing exactly that.

I'm hoping there's an easy way to do this that doesn't require a ton of coding. It seems like a feature that should just be built in to Unity honestly but I can't find a way to do it.

What I have:

What I would like to have: