Yup @ffco is correct, BUT be careful. The value you use may be different depending on the software you use.
My glass bed is about 7mm thick and my printer Z height is 216mm and I do not use Cura to print, I use Repetier Host
With Cura my z-offset statement is G92 Z208.795.
But if I was using Slic3r (this was about 3 years ago so it may have changed) I would have used the inverse i.e. G92 Z7.205.
Get it wrong and there is a loud clunk when the bed hits the extruder nozzle.
I've been able to do this by putting in starting gcode in the machine settings. Put in a G92 with your adjustment. For example, this moves my head .1mm from the build plate:
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