How do I do it?

i need to export animation as two passes, background and characters with an alpha.

When rendering the characters pass the pngs are with black background no alpha. I can't find any option on how to turn it on.

How do I render to png sequences with alpha?!

If not possible I'm in deep trouble right now having to deliver this weekend.

Render -> under "Generic" -> Transparent (version 19.08.x)