There are 3 ways:

1. In the general page of the video properties window, check "Keep ratio"

           

2. Click "Scene" in the right sidebar, click "Multi Screen", select "vert2" or "vert3" template, drag and drop to the waitting area, and drag and drop the 16:9  source to the sub window of multi-view.

         

      As shown below:
               

3. Use "Split Screen" mode, click  "Split 3" button, delete left and right split screen, only keep the middle part.