Direct debit reference?

I am filling out a direct debit from for my professional body as I have just graduated uni and on the form there is a section for a reference number. it is 8 characters long and I have already filled in the boxes for account number and sort code.

does anyone have any ideas
thank youu

Surely this is something they will need to provide, as they will be receiving the number and will need to know which membership(?) it supports. Look at any supporting documentation?