question

By the way, the call works fine from postman.

 void ALoginToPlayfabWithSteam::Login()
 {
   ClientModels::FLoginWithSteamRequest request;
   FString ticket = FString::FromHexBlob(static_cast<uint8*>(pTicket), pcbTicket);
   request.TitleId = FString("2C1DE");
   request.CreateAccount = true;
   request.SteamTicket = ticket;
   PlayFabClientPtr clientAPI = IPlayFabModuleInterface::Get().GetClientAPI();
   clientAPI->LoginWithSteam(request,
       UPlayFabClientAPI::FLoginWithSteamDelegate::CreateUObject(this, &ALoginToPlayfabWithSteam::OnLoginSuccess),
       FPlayFabErrorDelegate::CreateUObject(this, &ALoginToPlayfabWithSteam::OnLoginError));
 }
    
 void ALoginToPlayfabWithSteam::OnLoginSuccess(const ClientModels::FLoginResult& Result)
 {
   if (GEngine)
   {
       GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Green, TEXT("Successfuly logged in"));
   }
 }
    
 void ALoginToPlayfabWithSteam::OnLoginError(const FPlayFabCppError& ErrorResult)
 {
   if (GEngine)
   {
       GEngine->AddOnScreenDebugMessage(-1, 5.f, FColor::Green, TEXT("Login error: ") + ErrorResult.ErrorMessage);
   }
 }

I managed to solve it by adding the titleid to the endpoint directly. I'm not sure whether or not it is the intended behavior but it seems like adding it to the request does not change the endpoint.