That's the kill switch. Designed so you can flick it back to run to go get the MOB if needed. Here's the kill cord you need. Flick the switch "off" then catch the hoop by inserting it into the run area and switching the switch back up.
Wrap the other end around your leg/wrist. Please don't be tempted to ever run without the cord or with it connected to the motor rather than you! it won't "kill" the engine if you need it too
Personally I'd go genuine mercury as its rope rather than cord which stretches and stresses.
First pic cord you need.
Second cheapest available
Third the genuine mercury one I'd suggest is better value for money.
Other option is search for black dog marine as they give Ribnet 10% discount and you can order online.
Peter @ Boatsandoutboards4sale ~ email@example.com
~ 07930 421007