I'm afraid this solution didn't work at all for me. I've got a piece of line art with a white background. Absolutely nothing I have done will separate the background. I've followed video tutorials, have the background set to Transparent (as evidenced by the gray checkerboard on my screen). I've rasterized, selected my color (black), punched the "Mask" button in the Layers toolbox... and although it creates a new masked layer, nothing actually changes. The white background never disappears to reveal the gray checkerboard. The exported PNG, despite selecting "Transparent background" and only the Selected Item, still comes back with a black image on a solid white background.
In Publisher Pro, which I swear was a progenitor to this package, it was a simple two click operation. It literally had a button called "Select Transparent Color". I've been working with Affinity for two years, and this has been my constant, brutal frustration. I will literally walk to another building to fire up my old PC and use Publisher Pro to make a background go away because I simply cannot figure out how to do it in Affinity.
Help me Obi Wan Kenobi. You're my only hope.