3v3 pin how to read voltage

Due to some issue with battery plug i decided to use 3v3 pin for battery in my ab01 heltec’s.
Can anyone help me with code how to read battery voltage from that pin?

Maybe follow the link in this response:

You can’t read the voltage from that pin directly so, if you’re going to bypass the internal battery management circuitry (by not using the JST1.25 battery connector), you’ll have to build something similar externally.