If you did want to connect the external pack so it turns on/off with the board you'd want to get a BMS that will turn on with a momentary input since the power button on the OW isn't toggle/latching and is instead momentary. If you could get one then you could in theory connect up the thin blue wire and either of the larger black wires to trigger the external BMS. I briefly went over tricking the OW BMS to turn on with this method in another post.
I saw your video before, i did not know it was you, thanks !
Yes that is exactly what I want to do.
Most cheap BMS don't have switch, some do, and I feel it will be difficult finding one compatible with the proprietary FM 60V shorted to GND pulse.
Maybe the way is to add a "low pulse trigger" latching relay to a BMS with a switch :
either directly on blue pin #1, either through a resistor divider to lower its 60V voltage.