You can contact their support staff either on the site (but you are restricted so that would probably be a no go there) or by email.
I had to contact them only on one occasion so I will need to look for that address for you.
There is sportsbet.io representative by the name of steve here on the announcements thread.
But I do not think he deals with customer support issues to do with account balances.
He could steer you in the right direction.