Magnetos button, where?

I'm trying to build a small panel for the starter key and som switches for the cessna c172sp in fsx. But I can't find the event thats buts the starter key to "magnetos both" position. I can get the starter going but when I release that button the key flips back to the off position. I have tried the m-key on the keyboerd (magnetos select) but nothing at all happens then.Please helpTony W