How do I select seats on China Eastern?

[suze2000]

Just booked a flight from PVG to PEK directly through MU (on the C919).

Go to ceair.com

Using the MyTrip (NOT seat selection) page, plug in the ticket number (starting with 781, not the PNR), your surname, your first AND middle name (if your middle name is on the booking), your flight number (WITHOUT MU) and your ID document (probably your passport number).

Yeah it won't work. I think the passport number hasn't been added to the booking. I used a TA but starting to wish I hadn't.