There might be an easier way, but the way I do it is by using the BPR function.
If you do a BPR render (the little button on the upper right of the default layout, keyboard shortcut Shift+R), you can go into the Render -> BPR Renderpass menu and click on the "Mask" pass, allowing you to save that mask out for future use.
Take that and the final render into Photoshop and use them to create a transparent image.
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If you do a BPR render (the little button on the upper right of the default layout, keyboard shortcut Shift+R), you can go into the Render -> BPR Renderpass menu and click on the "Mask" pass, allowing you to save that mask out for future use.
Take that and the final render into Photoshop and use them to create a transparent image.