If you pull the blu/wht connection up to B+, on the Andersen connector, through a 100K resistor, it will close the contactor if there is some residual voltage left at the B+, even just a couple of volts. If the bike has been sitting for long then you may have to momentarily turn on the key until the contactor closes to charge the internal capacitor, to provide the residual voltage. Usually you won't need to do that.
When you connect the 100K line and you hear the contactor close, then power up your external charger. If the charger voltage is present first, the contactor won't close.
I had to figure this out to add built-in Meanwell chargers to my e-trike conversion. I also added another smaller connector for those (they only provide 6 amps) so that I could disconnect the charger after charging and I can still use the Andersen connector for an additional external charge at the same time.
The BMS will terminate the charge when it is finished so I set my charger at 116 VDC (actually 115.8 VDC)
Trikester