Save GIF with a transparent background

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Principle Version: Version 5.12 (5024)
macOS Version: 10.14.6
Sketch Version:
Description of what you need help with:
I added a video into a mobile mockup on the principle and would like to explore a GIF with a transparent background, but I couldn’t find any way to do that. Can anyone help?

Steps to reproduce the behavior:

Screenshots/Sketch File/Principle file: (if applicable)