There is no one specific function, but rather a series of functions that need to be performed. Cropping a video is not the same as cropping a photo. Not at least in what you want to accomplish.
The first step is setting your Video Mode. You’ll need to determine what size you want your video to be. There are presets to select, but it appears your specific selection is not one of the presets. You will need to create a custom video mode.
Next is using the Crop: Source filter.
Making the red outline required the use of another video track above the source video, a color source, and the filter Mask: Simple Shape. Shotcut has an internal color source you could use (Open Other → Color).
Example:
With the filter deselected to see the preview better.
