How do I sign up for water with a Harris County MUD?

I'm moving to the burbs and will be in MUD 18.

I can't figure out how I am supposed to sign up for water service? It seems I can register via eaglewater.firstbilling.com, but it also seems I need to already know my account number?

Will I just have to look for a water bill in the mail with "my" account number and then sign up? First time home owner and I've never had to sign up for water before. Thanks in advance.