They recently made a video showing what it is, but basically:
Log in to your account at the Wind Mobile website, and you should see your account ID underneath the "Your account" heading on the right (probably starts with DBC).
This is the account ID that you use. I just tried it with TD.

I think that it is something like 1234 DONT 456 USE 789 WIND 999 MOBILE but you will have to double check with their rude customer service.

Hi Richard and Maurice. Please contact Wind Mobile to find out the details for paying electronically via RBC bank.