How to make a In Game Add On

I made a free game to play, which your source of money is through purchases of “Skins”. I made a purchase page of it, all cute, but I couldn’t find any Tutorial on how to get game buyers to a purchase page.

An example is the “Rocket League” Game: When I click on a Buy Credits Button (

) it directs the player to the Unreal page to make the purchase (