How to use the anti-static wrist strap

Usually the center screw or the ground outlet on an grounded electrical outlet. Any water or gas pipe.

"If you want to ground yourself you will have to connect to a grounded part of an outlet, waterpipe or something metal that is grounded.

The chassis of the computer would work for grounding if it would be plugged in. However it is not recommended to have a device plugged in while you are working on it so it is probably best not to have if plugged in.

Another thing to note here is that it is actually the difference in charge between you and the internal part of the computer that will create the discharge. The discharge naturally happens to equalize the charge. So you can still use your computer chassis (even when not plugged in) to even out the static charge between you and the chassis and eliminate the risk of electrostatic discharge happening."