Because it is not possible to directly upload video files to Designer, you should link to them on YouTube instead. To do so, follow these instructions:
Please note that you have to modify the code here.
The copied link in the example is: 4AqIYM9TkF8
The example link entered into the editing area is as follows:
<iframe src="//www.youtube.com/embed/FHtvDA0W34I" frameborder="0" allowfullscreen></iframe>
Replace the video identifier (FHtvDA0W34I) with that of the video to which you are linking (here 4AqIYM9TkF8) and then click «Save».
The view is updated immediately and the correct video is displayed as a «picture placeholder».
Click now on «Preview», you can check whether the desired YouTube video is correctly embedded and displayed.