HP 4050TN ,the black plastic arm that drops down into the middle of the paper tray .

Hi,

I am looking for some advice regarding how to repair my printer (HP 4050TN) - the black plastic arm that drops down into the middle of the paper tray (I think it is a paper sensor) came out of its place - it does not seem to be damaged in any way, but I cannot put it back in the printer - I checked the HP website and they have a "print path simulator" animation that shows the insides of the printer - the part that I have removed is PS101, from paper tray number two.

Does anybody know how to put this arm back into the printer, please?