I have a Mac keyboard (actually a third-party keyboard) and I remapped the ST Help key to F13 on my keyboard. I realize the standard Mac keyboard does not have an F13.
I used the keytest.prg program in tests/keymap folder (when you download the source) to determine the correct scancode to enter into keymap.txt (default name that Hatari always loads for the keymapping). This program allows you to hit any key and it shows you the value that is generated on the ST. Keymap.txt is put in the same directory as the Hatari application package.
# 294 = F13, 98 = ST Help key
I looked in the Hatari source code to find the hex value for the Help key. Looking at the code, it is 0x62 (98 Decimal). Since I needed a decimal value, I did a simple conversion and came up with 98. I commented my keymap.txt file in case I ever had to do this again.
There may be an easier way to do this, but it worked for me.