First, you need to do this in the Timeline Mode. There are handles on the video/audio objects. I am loading a picture illustration here to show the handles
With the fade out handle, you left click, hold and drag to the right. You will notice a small line diagonally connecting the handle to the corner, this is giving you a visual of the where the fade starts to go to black in a time reference.
I'm glad you were able to solve your problem but actually, the procedure has been exactly the same as the solution given here for as many years as I have been using Magix products! It certainly hasn't changed at all, to my knowledge. 😃