So you need to fit a switch to send live to ign / glow plugs ( this must be a latching switch )
then fit a PTC ( push to contact) switch to starter circuit.
You can buy both of these a panel on ebay etc ( like an aircraft switch with cover, with a push button next to it)
Ign is same as pos 2 on key switch
PTT is same as pos 3 on key switch
In the most basic installation, All can be wired direct to switch at rear of key barrel on steering column, (i would use a relay for the starter button due to the high current required)
Depending on the model of defender and year as per the wiring diagram Exampe Pic below you dont need the second switch in the pic just the first and second from left