Air China/Flightcatchers ID number

So I booked a flight with Air China from the UK to Tokyo through Flightcatchers. But I'm struggling to find my 'Passenger ID Number' to confirm my booking to make sure I haven't been scammed, and mainly coz I'll need it to check in to the flight.

I rang up air china today and they said that they can see the ticket on their end (so it's not a scam), but just for my peace of mind I want to confirm it on my end - does anybody know where I can find my passenger ID Number? I have access to all of my other information for the flight, just not that one.

Thank you!!

Edit: I googled this before and it said to use my passport number - I did and it didn't work so it's not that :/