Press and hold in various buttons until the right thing happens?
Find a 12 year-old boy?

Park next to a similar Polo and ask the owner how to do it.  They may at least
have an owner's handbook.

I was at a filling station refuelling my 15 year-old Polo and a couple came over
and said they were having trouble with their (much newer) Polo fuel filler cap.
I think they had it partly unlocked, as mine won't move in that situation.  I
just unlocked it, took out the key and it undid no problem.

BTW, VW dealers often have odd copies of recent model handbooks - be cheeky and
ask.  They may get your business subsequently.