Removing Oxbox GripTip
"Drain knockout is easy to remove with pliers" –siouxchief.com

No they're not. You need to use a utility knife. This should have been done before it was installed. I'm unfamiliar with that type, because it looks like it has a stub that you can cut open, shove the hose over it, and use a hose clamp. (?)

Usually you just cut it out, tracing the inside of the circle, and shove the hose down in there. You're sure there's a (2") drain pipe connected to it inside the wall, right? DO NOT over cut the circle or you'll ruin the box. Start small if you're unsure. Be careful; don't cut yourself.