Button on Linuxcnc interface that switches an output on Mesa 7i76e on and off

You'll need to make the hal connections in the postgui_hal file. Have a look at the millturn sim confi, (uses an xml instead of a glade file but the setup is the same.
github.com/LinuxCNC/linuxcnc/tree/master...xis/vismach/millturn

That does not work because you have forgotten to give a signal name after the 'net' command.

I entered the command in postgui. This does not start linuxcnc. Do I have the correct command from Halshow for postgui.hal?

Not sure I understand this correctly but halshow does not make any distinction between postgui and regular hal files.
The regular hal files are read before the gui has loaded so if any gui related halpins are used in these files you will get a startup error because those pins are missing. The postgui halfile is read after the GUi and the custom panels have loaded.

Note that you can use either a handler or the postgui halfile to make the connections.

If your Glade UI element is of type "Hal button" then the HAL pin will be created automatically and no handler file is needed.

(This is stated with 93% confidence, I don't have easy access to a LinuxCNC machine at the moment to test this assertion)