Roto to Alpha matte workflow

Can anyone give me a brief description of the procedure to extract an alpha matte from a roto. I'm sure it is rather simple. I am a DP that is trying to learn fusion. I am running fusion 9 studio and have become successful at pulling a good roto but I am struggling trying to create the alpha matte. I recently shot a horror short at night. During one conversation piece (typical OTS shots) the background is primarily dark black. I am trying to use the matte to add trees and depth to the shot.

Thanks,
Eric

Pipe your roto to whatever "...Key..." node you'll feel comfortable with...

I personally use "Primatte Keyer" because I was used to in other pieces of software, but search for "keyer" word among the tools and learn to use it, you'll have your alpha then (which you'll be even able to fine tune adding an Matte Control after).