Is it possible to remap modifier key + volume up/down?

I'm trying to modify control + volume down/up into different keys (vol up/down is f12/f11). Is this even possible? When I try, it defaults to "volume up/down", removing the "control" key, which isn't what I want.

I'm basically trying to set "control + vol down/up" to "f18/f19", keys that don't exist on my board. And then remapping f18/f19 to scroll up/down. Basically I'm trying to make it so that I can use my volume knob as a scroll knob if I also hold down "control".

I remapped this before with Karabiner and it worked fine but suddenly stopped working. I'm trying to figure out if I can replicate this in BTT, but I'm not finding any success.

That should work fine: