It should work when setting the background to black and then rendering as PNG

Go to Project settings and then under 2D Background, make sure Background Color is black and Active Image is disabled.



I just tried it and it still works. However, some image viewers don't show transparency, so you need something like Paint.net or Krita, or Photoshop to verify.

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Strictly speaking, the background color does not matter as long as background image is unchecked.
You set a background color at will and it would affect the output object contour matting and visual of semi-transparent parts of the object (if you have it on the character).
I'd say set the background color to any gray scale value which would correspond to an overall lightness of the background to be used with your rendered object.