Is it possible to configure Steam microtransaction using blueprints on UE4?
No, the Steam OnlinePlatform plugin does not have it implemented. The reason for that is that Steam microtransactions are very different from the paradigm the OnlineSubsystem is designed for, which is for stores where the developer registers products ahead of time with IDs and prices.
Steam, on the other hand, works more like a payment processor when it comes to microtransactions. For starters, you must have a web server of your own that communicates with Steam via the SteamWorks web API, as detailed here: Microtransactions Implementation Guide (Steamworks Documentation)
On client-side, there’s very little interaction you need to do. There’s a callback you can hook to to detect when a transaction was completed but that’s it: the bulk of the work is done between your backend server, the steam servers and the Steam overlay.
Thank you for your answer.