special relay idea needed

I would like to make a device that I can have a circuit active only when my ignition is on, but it has to be initiated by a momentary switch, which then allows the device to be active when the ignition is on. Here's the procedure to activate it: 1. Hold the momentary switch in the "on" position 2. Start the vehicle 3. Let go of momentary switch and device stays active 4. Device stays active until ignition is turned off. ***Actions 1 to 3 required again to activate the item. Can anyone tell me how to make this work with relays or ??? Thanks for your help

https://www.the12volt.com/relays/page5.asp#lspYou will have to substitute the wire labeled Constant 12 volts (In the picture) With a true ignition source.  You will have to turn the key to the on position before you press the momentary switch.  The relay will then stay on through the start cycle and remain on until the ignition switch is turned off or the switch is pressed again.

Fabulous, thanks!

Amazingly we call this device an accessory lead.

You have to use a True Ignition source to power this apparatus.  An accessory wire loses voltage during a start condition.  A true ignition is hot when the ignition is  ON and it is still on during the start cycle.