I have a Bolens riding mower and when not seated and parking break is engaged the engine will shutoff.

Dead mans switch under the seat. So the mower won't run over you can make you into hamburger when you fall off after a few too many beers mowing the yard.

Or it could be a bad switch that disables it when the mower is in neutral that some have. On my Craftsman, the deadman switch is active any time the mower deck is engaged and spinning, parking brake set or not.

Where is this switch located on a Bolens model #13AC762F065

You are going to have to wait for someone with more specific knowledge of that mower for that info. These exist on all mowers as a result of federally mandated safety features. In case you ever wondered why push mowers have that engine brake that shuts it down quick if both hands leave the push bar. Same reason.

Sounds like the switch is working and doing its job. Finding should be just a matter of following any wires attached to the seat. Probably not too many.
Disabiling or bypassing will make the machine unsafe, a potential lawsuit should someone get hurt.
Good Luck
Bypassing is sometime necessary for troubleshooting, take precautions to ensure safety. You can loose parts, and not from the machine.

The problem is when the mowers parking break is engaged and the blades are NOT engaged the engine should idle instead of shutting off when you need to get off the mower to pick up trash, etc.

Has it allways been like that, or just started?