[Use custom shadow channel to remove background]

I 3D rendered an image with a shadow channel, I am trying to remove the background of the rendering with the shadow channel so that the background is removed but the shadow remains. The screenshot outlines what im trying to achieve with photoshop. Is this possible with Affinity? If so direction on how to do so would be greatly appreciated 

Perspective1.afphoto

[How to paste an alpha mask into the channels]

I 3D rendered an image with a shadow channel, I am trying to remove the background of the rendering with the shadow channel. The screenshot outlines what im trying to achieve using photoshop. Is this possible with Affinity? If so a direction on how to do so would be great!