How to get transparent rendered background when using HDR in Flamingo

There is very probably a simple answer to this…

I have sent up a new environment with an HDR image of a sky scape that I want to be reflected on the model. That is all working fine.

However, when I render I would like a transparent background and not the HDR image. How do I do this?

(The aim is to compose the final image in PhotoShop.)

Thanks for any help.