My thinking is, you'd close the cap (screw the cap on) without the key in the cap. Before inserting the cap, put the key in and turn the key clockwise (CW), then remove the key. Put the cap into the filler neck and twist until the cap is sealed. Once all the way down, try to unscrew (without the key). I'll bet it won't come off. To pull the cap off, put the key in the slot and turn it counterclockwise (CCW) and unscrew the cap. You may be able to turn the key, then take the key out, then unscrew the cap.