How to write a macro to Set work zero

no, avoid G92
The correct code is 'G10 L20 Ps X. Y. Z' as in G Codes
You will need 3 macros, one for each axis
for X

Code:

G90 G17
G10 L20 P0 X0

for Y

Code:

G90 G17
G10 L20 P0 Y0

for Z

Code:

G90 G17
G10 L20 P0 Z0

You could combine X and Y but I don't recommend combining with Z
for XY

Code:

G90 G17
G10 L20 P0 X0 Y0

The 'P0' tells it to use the current work offset system, which avoids having to know which one is in use, though G54 is the default.

Then for the gotos I would use 2 macros
first, goto X0 y0 and a safe Z height

Code:

G90 G17 G21
G53 G0 Z-5
G0 X0 Y0

That sets millimeter mode then raises Z to 5mm below the Z limit switch. IF that is not far enough to avoid triggering the limit, then use -10
G53 is a machine coordinate move, referenced to home rather than workpiece 0, G90 is absolute coordinates (as opposed to relative), G17 is XY plane, G21 is millimeters.
Macros must always set the modes they expect, never assume that the mode, like G90, is already in effect, someday, it won't be )-:

now for Z, I would not rapid move to Z0, that would not give me time to abort if there was a problem

Code:

G90 G17 G21
G1 Z0 F400

This sets the modes as before, then uses a feedrate of 400mm/min to move to the current work Z0.
This will do 100mm in 15 seconds, which should give you time to emergency stop if there is a problem, without taking so long you get bored.
Of course you can adjust this feedrate to be faster, your machine, your risk, I find fast Z moves to happen much too quickly for comfort so I take the slow road.