If you install Thonny and run the terminal monitor, the API's for any device connected will be shown in the terminal for Micropython.

By using this code, I was able to get the API Key:

import deviceCfg
akey=deviceCfg.get_apikey()

BTW, the MAC address can be obtained with the following code:

macaddr=wifiCfg.wlan_sta.config('mac')
macaddr='{:02x}:{:02x}:{:02x}:{:02x}:{:02x}:{:02x}'.format(*macaddr)

Thank you!

import deviceCfg
print("APIKEY: " + deviceCfg.get_apikey())

But the APIKEY not fixed.I think you should use MAC addr for the devices:

import machine
import binascii
mac_addr = binascii.hexlify(machine.unique_id()).decode('utf-8')

I tried the following code, but I get an error.

from m5stack import *
from m5ui import *
from uiflow import *

setScreenColor(0x111111)

print(APIKEY)

"_APIKEY" also resulted in an error.

Do I need to install any additional modules?

print(APIKEY)
#or
print(_APIKEY)

Should work. On device boot in serial monitor was displayed all boot messages includes apikey.