Octagon box wire access point
To use the knock out in the middle of the box you Need to use a lock nut type clamp.
You remove the lock ring from the clamp and pass the threaded part of the body through the knockout from the outside and then place the lock nut on the threads from the inside of the box And tighten.
Then you can thread your NM Cable (Romex) through the clamp and tighten down the clamp screws.

This  article will explain it.