Hiya Big Chevy was thinking of doing this myself so I looked it up a while back and found this hope it helps.
Take one DPDT relay.
Connect your ignition line to the microswitch.
Connect the microswitch to the coil.
Connect your coil to ground.
Now connect your ignition to the NO (normally open) on one of the relays contacts.
Connect the COM (common) for these contacts to the coil.
The other set of contacts will operate your load (or a bigger relay).
When you turn the ignition on the relay will not operate until the microswitch is pressed. Once the microswitch is pressed, the relay will keep itself powered until the ignition is turned off again
