Electrical cord to gearbox disconnected, how to reconnect it?

My husband accidentally disconnected the electric plug to the gearbox (light blue plug) and we cannot figure out how to reconnect it.
When he unplugged it, he had pushed up on the blue plastic (says open). We've tried pushing it back where it originally was but it does not "click" in and connect/stays in place.
How are we supposed to reconnect this?
Should we be pushing on the blue plastic to be on the "open" while connecting it? We have very little space to maneuver with our hands and cannot take the rest of the box off due to other cables being connected.

Thank you for the help!