[2019] Infected - how do I use restock perk?

I love infected mode. The only thing I can't figure out is how to use the ammo restock? When I kill a couple of zombies I get the perk and it's shown on the right hand side of my screen but I can't seem to be able to use it? I've tried pressing the "killstreak" keys but it doesn't work.

Also, does infected mode count towards my general K/D ratio?