I took a parcel to my PO before lockdown. It was for someone I don't know particularly well, and I didn't know her postcode offhand. I wrote out the address, intended to look up the postcode, but was distracted when the phone rang, and forgot.
I didn't realise until I got to the PO, when the counter clerk asked if I knew it. I was about to stand out of the queue to look it up on my phone when I realised that she had looked it up for me and added it to the address.
In the circumstances, I don't think it was expecting too much for the manager to fill in the form for you, if it was a short one. Did you ask him to?