Creating DIY ON/CHARGING switch for an Arduino
Q2 is a mosfet used to disable power draw from VBAT when 5V or VUSB is present. Disconnect the trace from pin 3, and place a switch between pin 3 and the trace from VBAT.

Based on what I see on the schematic. This means it can still charge from 5V. But will only power the 3.3v regulator when the switch is on.

Edit: based on https://macsbug.wordpress.com/2017/05/08/battery-and-battery-interface-of-lolin32/ there are two revisions. The original uses a diode or bridge, while the newer uses a mosfet there. The same applies to either revision. Disconnect the VBAT from q2 or the diode bridge and insert a switch.

Edit 2: here https://macsbug.wordpress.com/2017/05/31/power-switch-to-wemos-lolin32/ they mod a power switch. Similar to what I suggested, this would also keep the board mostly off even of you use a usb or 5v power source. It will allow you to charge and have the board off, but not board on no charging.