Are you able to log in to My Account. Your account number is shown there.
Otherwise a call to Optus.
That's the problem, I need the number to create My Account. Optus did not send me any info regarding my broadband detail: no welcome pack, no email. I did not even know the landline phone number link with the account
Online chat to Optus then.
Just a thought have you checked your spam emails?
Another thought by the sound of it you haven't received your first bill yet, this will have your account number so wait for this to arrive. You have to pay this first before you can set up direct debit.