Open up your code solution, search for “CreateWidget” (or use ALT+SHIFT+S if you have Visual Assist installed), you should see it pop up UserWidget.h. Same with MediaPlayer/OpenSource/etc.
In the end you end up with something like
UIntroWidget* NewIntroWidget = CreateWidget<UIntroWidget>(GetWorld(), UIntroWidget::StaticClass()); // Class name will be different if you made it from a Blueprint, something like UIntroWidget_C or some such.
UMediaPlayer* MyMediaPlayer = NewObject<UMediaPlayer>();
MyMediaPlayer->OpenSource(IntroMovie); // Assuming you have UMediaSource* named IntroMovie like in your BP.
Blueprints are thin wrappers around C++ code. So if you have a blueprint, you can find out exactly what C++ it is using just by searching through the code a bit. If you aren’t building the UE4 Engine from the Git source, I highly suggest you do it - even if you don’t plan to modify it as being able to read the code and walk through things in a debugger is invaluable.