SteamCallback not working

Hi,
I am currently working on Microtransactions with Steam. The serverside Part of the whole transaction process is already working and currently i need to implement the SteamCallback OnMicroTxnAuthorizationResponse after I click “authorize” in my SteamOverlay. However the Callback is not working at the Moment, this is how I got so far:
Header file:

#include "steam/steam_api.h"
#include "steam/isteamuser.h"

/**
* Controller UCLASS
*/

class SteamCallbacks{
public:
	SteamCallbacks();

	STEAM_CALLBACK(SteamCallbacks, OnMicroTxnAuthorizationResponse, MicroTxnAuthorizationResponse_t, m_MicroTxnAuthorizationResponse);
};

CPP File:

SteamCallbacks::SteamCallbacks()
	: m_MicroTxnAuthorizationResponse(this, &SteamCallbacks::OnMicroTxnAuthorizationResponse)
{};

void SteamCallbacks::OnMicroTxnAuthorizationResponse(MicroTxnAuthorizationResponse_t *pCallback){
	GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Red, TEXT("CALLBACK RECEIVED"));
}

This Class is in a PlayerController which is parent to a BlueprintController which is DefaultController in a shop only Gamemode. In addition im calling SteamAPI_RunCallbacks() every frame from a blueprint library.
I do not know what I am missing to make the callback work and neither the Steamworks Documentation nor the example project are helpfull.

I hope someone knows how to fix my issue.

Kind Regards
Alex

bumping this to the front overview … since we want to avoid the web-based process, preferring the ingame solution.

Try use STEAM_CALLBACK_MANUAL instead.