There are 3 ways:
- In the general page of the video properties window, check "Keep ratio"
- 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:
- Use "Split Screen" mode, click "Split 3" button, delete left and right split screen, only keep the middle part.