Blueprint Function Library を継承したクラスを作成し関数を追加します。

UGlobalBPFunctionLibrary.h

UCLASS() 
class GAME01_API UGlobalBPFunctionLibrary : public UBlueprintFunctionLibrary 
{ 
    GENERATED_BODY() 
 
public: 
    UFUNCTION(BlueprintCallable, Category = "GlobalBPFunctionLibrary") 
    static void SetupGameWindow(); 
};

UGlobalBPFunctionLibrary.cpp

void UGlobalBPFunctionLibrary::SetupGameWindow() 
{ 
    if (GIsEditor) 
    { 
        return; 
    } 
 
    HWND hWnd = NULL; 
    TSharedPtr<SWindow> MainWindow = GEngine->GameViewport->GetWindow(); 
    if (MainWindow.IsValid()) 
    { 
        if (MainWindow->GetNativeWindow().IsValid()) 
        { 
            hWnd = static_cast<HWND>(MainWindow->GetNativeWindow()->GetOSWindowHandle()); 
        } 
    } 
 
    if (hWnd) 
    { 
        LONG Style = ::GetWindowLong(hWnd, GWL_STYLE); 
        Style &= ~(WS_THICKFRAME | WS_MAXIMIZEBOX); 
        ::SetWindowLong(hWnd, GWL_STYLE, Style); 
        ::ShowWindow(hWnd, SW_SHOW); 
    } 
}