Depends on how the gate drive works in the BMS, but if it can source enough current, you can simply disconnect the gate on the FETs, then run the gate driver output to the positive of the relay coil, with the negative of the relay coil to battery negative.
What voltage the coil needs to be depends on the voltage out of the gate driver.
If the gate driver cannot source much current, you can have it drive an NPN transistor instead, and put the transistor in the negative side of teh coil (between the coil and ground), and run the positive of the coil to battery positive (using a relay with a coil voltage the same as your pack voltage), or run the positive to the power supply of teh gate driver (with a coil voltage the same as whatever that is, and assuming that power suply can supply the coil current).
Some info on driving relays; it's pretty simple
https://www.google.com/search?q=driving+a+relay&ie=utf-8&oe=utf-8
Then use the relay contacts to break the connection of the keyswitch wire to the battery positive. The contacts must be rated for DC, and a minimum of whatever the fully-charged pack voltage is. They must also be rated for the maximum current the keyswitch line sees, so if you're powering a DC-DC from that as well, make sure whatever it's peak current is is taken into account.
NC contacts will be normally-closed, meaning when the relay coil is NOT powered, the battery will stay connected to the keyswitch.
NO contacts will be normally-open, meaning the battery will be connected from the keyswitch only whenever the relay coil IS powered. (and disconnected whenever the relay coil is NOT powered).
So it depends on which way you design the coil drive circuit which contact set ot use, but I'd recommend using the NO set, so that when power fails at the battery for whatever reason, it disconnects the keyswitch from it, making it unable to turn on the bike under that condition.
FWIW, this setup is going to disconnect the keyswitch power under *any* condition that triggers the BMS shutdown, including overcurrent if the BMS monitors that. If you don't want that to happen, you'd need to defeat the overcurrent protection of the BMS (like by replacing the shunt in it with a big fat wire).