Myblueprintfunctionlibrary.h:
// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "CoreMinimal.h"
#include "Kismet/BlueprintFunctionLibrary.h"
#include "Engine/GameEngine.h"

#include "MyBlueprintFunctionLibrary.generated.h"

UCLASS()
class MP_SK_API UMyBlueprintFunctionLibrary : public UBlueprintFunctionLibrary
{
    GENERATED_BODY()

public:

    //const FText& NotString
    UFUNCTION(BlueprintCallable, Category = "Notification")
    static void AddNotificationToScreen(const FText& NotString);

};
Myblueprintfunctionlibrary.cpp:
// Fill out your copyright notice in the Description page of Project Settings.

#include "MyBlueprintFunctionLibrary.h"
//#include "SNotificationList.h"
#include "Widgets/Notifications/SNotificationList.h"
#include "Runtime/Slate/Public/Framework/Notifications/NotificationManager.h"
//#include "Framework/Notifications/NotificationManager.h"
#include "Engine/GameEngine.h"

void UMyBlueprintFunctionLibrary::AddNotificationToScreen(const FText& NotString)
{

    UGameEngine* engine = Cast<UGameEngine>(GEngine);

    if (engine)
    {

        if (!engine->GameViewport->Viewport->IsForegroundWindow())
        {
            //const FText NotText = FText::FromString(NotString);
            //const FText NotificationText = FText::Format(NSLOCTEXT("MixerInteractivityEditor", "DesktopNotificationAlert", "{0}" ), NotText );
            //FNotificationInfo info(FText::FromString("A Notification"));
            FNotificationInfo Info(NotString);

            Info.bFireAndForget = true;
            Info.FadeInDuration = 0.2f;
            Info.FadeOutDuration = 1.0f;
            Info.ExpireDuration = 8.0f;
            Info.bUseLargeFont = true;

            /* we define our callback lambda
            auto notificationClicked = [](const FGeometry &, const FPointerEvent &) -> FReply
            {
                // we restore the game window no matter where it is
                UGameEngine* engine = Cast<UGameEngine>(GEngine);
                if (engine)
                {
                    TSharedPtr<SWindow> windowRef = engine->GameViewportWindow.Pin();
                    SWindow* window = windowRef.Get();
                    if (window)
                    {
                        if (window->IsWindowMinimized())
                        {
                            window->Restore();
                            window->ShowWindow();
                        }

                        window->BringToFront(true);
                        window->HACK_ForceToFront();
                    }
                }

                return FReply::Handled();
            };*/

            //TSharedPtr<SNotificationItem> item =
            FSlateNotificationManager::Get().AddNotification(Info);

            /*
            if (item.IsValid())
            {
                item.Get()->SetOnMouseButtonDown(FPointerEventHandler::CreateLambda(notificationClicked));
            }

            // we force the notification window to the foreground
            TArray<TSharedRef<SWindow>> windows;
            FSlateNotificationManager::Get().GetWindows(windows);

            int32 num = windows.Num();
            for (int32 i = 0; i < num; ++i)
            {
                FSlateNotificationManager::Get().ForceNotificationsInFront(windows[i]);

                SWindow& win = windows[i].Get();
                win.Restore();
                win.ShowWindow();
                win.BringToFront(true);

                win.HACK_ForceToFront();
            }*/
        }
    }
}