Ffmpeg and hevc_nvenc - how to output hevc with alpha channel transparency?

The documentation states that nvenc supports alpha channel with hevc.

https://docs.nvidia.com/video-technologies/video-codec-sdk/nvenc-video-encoder-api-prog-guide/#hevc-alpha-encoding

I cannot find any way to use this with ffmpeg. How can I generate an hevc video with alpha channel using nvenc?

Here is an example of an hevc file with alpha transparency https://doggo.s3.amazonaws.com/output.mov

I want to encode a sequence of PNG or WEBM files with alpha transparency into an HEVC video with an alpha channel.

Thanks!