When I turn the key to start my 2018 500 H.O. it will not stay perfectly in place to kick on. I physically have to hold the key in the position to start it then when I let go it shuts off. I only moves maybe millimeter but that's enough to turn the machine off. Do I need to change the ignition switch?

I would say yes.

[2015 Z Force 800 53/Trail EPS]

I second that..yes.