Super Noob - Want to layer effects behind subject

Thanks in advance for helping me with this, I’m an experienced Pshop user (Win 10 laptop, PSshopCC) but very new to and excited about Optics.

I shot a model against a gray paper backdrop.

I want to take the pic into Optics from Pshop, use EZ Mask to cut her out, then put star fields, moon, clouds, etc behind her.

I start Optics from the Pshop filters menu. I add the S-Nightstars effect, mask her to keep the stars off her…then end up with my gray paper background covered in wonderful stars. How do I get rid of the backdrop but keep the stars? All inverting the mask does is put the stars on her and off the backdrop.

I’ve tried cutting her out in Pshop and bringing that into Optics but I end up there with a solid black (or white from the menu) background that I can’t do anything with.

Seems like I’m missing a simple switch or setting but I can’t figure it out.