Battery packs connect with a multipin connector.
There is (almost?) always a single/dual red wire and a single/dual black wire. That's power.
There can also be wires for almost anything else: 10 k thermistors, fixed resistors for ID, BMS ICs.
Depending on firmware, some devices refuse to boot without a valid battery.
What you do is take a bunch of tape off your battery pack until you see the tiny strip PCB that is on the top.
This attaches to the actual battery using the battery's foil tabs.
Break the tabs, recycle the leftover battery cell, solder your power suppy to the PCBs terminals.
If you have a good USB supply, try connecting that instead of your PS through a 1N400x diode to drop it down to a reasonable battery voltage.
If you have problems with it booting correctly throw an electrolytic on the 4.3 V side of the diode.