Here's an easy way to do it. I figured this out on my first Model A when I was 15, about a hundred years ago.
Take the starter switch apart, turn the copper contact strip upside down and re-assemble it to the starter. That provides a constant contact with the "button". Then you have to use a solenoid like Steve Preston suggested.
For me, this was a lot easier and cheaper than modifying the starter with a stud (although I know the parts are available) plus the fact that if you hide the solenoid, it looks original.