Your present throttle must have 4 wires: 5v, ground, throttle signal, all for the throttle operation; and 48v for the LED panel, which will take a spur of the throttle ground.
A throttle with voltage display and a switch will have an extra two wires for the switch. There are three ways to use it to disable the bike.
1. If you have no PAS, you cut the red wire to the throttle and join the two ends to the switch wire. That's the easiest, but won't work if you have a pedal sensor. If you have a PAS, you'd have to cut the 5v wires to both the PAS and the throttle, join one controller side end to a switch wire and the other switch wire to both the PAS and throttle 5v supply wires. This will leave the controller still switched on draining your battery at a very low rate - maybe 4W.
2. Find the ignition wire to the controller if you have one, cut it and join the switch wires to the ends. That switches off the controller completely.
3. If you have a battery switch, open the battery case, unsolder the wires and join them to the switch. That only works if it's a battery switch that stays in possition. Some of the press-button types are momentaru switches and only provide a signal to the electronics to change mode from off to on and vice versa. In that case, you'd have to switch your key switch on and off each time to mchange mode.