Looks like you have to add the account through your My Usage page.
The help / instructions can be found at https://www.agl.com.au/help-support/energy/manage-your-account
Click on the link and a new Web page will be started for you.
However, I can not find how to add your account to your My Usage page, it appears the first account needs to be added by the help desk. (You can add a second account though?)
To contact AGL go to https://www.agl.com.au/help-support/contact-us
Chat and Phone appear to be very busy at the moment, so be prepared to wait a while.
If you are sitting at a computer I think that using chat is better as you get a loud Ding when a message is sent, if you are wandering around the using phone support may be better as you can walk around while waiting in the cue.
I still would suggest that you create your My Account on the web if you have not done so before.
Hope this helps