The white "screws", are knockout plugs. A knockout is simply a hole where conduit, cable, wire, or a device can attach/enter the box. A knockout plug, is used to fill unused knockouts.

Here is a similar box cover with the knockouts unplugged.

This allows you to attach devices, like lamp holders and motion sensors.

Any extra holes would be filled with a plug.

Professional Help

You should be able to hire an Electrician to install a switch as you mention, but it would depend on if you have the legal right to (own vs. rent, etc.)

On the PIR, you are shorting V- in and V- out. Are you positive the documentation says they are bridged together? If so, you can omit one wire. If not, that's half your problem.

Also you are switching the inlet of the PIR, meaning you are severing power to it. PIRs often need to "get a sense of the lay of the land" which they then store and use as a reference for "unoccupied". If they also monitor day/night, it helps them to go through a couple of 24hr cycles to learn what your location's highs and lows are. These units do not have expensive NVRAM: If you sever power, it's like a factory reset.

So supply + and - to the PIR inputs at all times. Put the PIR output on one switch leg, constant power on the other, and to the LED in the middle. Whether you switch + or - should depend on what the PIR switches... check its docs. If you are not sure, get a DPDT and switch both.

If you can confirm that one of those wires is bridged inside the PIR, you can nut them all together, eliminate one of them to the PIR (redundant) and use a single-pole (SPDT) switch.