If you're billing using the same service location, you can set a default in your Account Settings so you don't need to enter this every time on your claims.
Access Your Account Settings
You can find your setting under your email address located on the top-right of the screen.
Province Specific Settings
Find the province you're billing under here and select Edit. If you bill in more than one province you'll see options for each province. Your settings are unique to each one, so you can have different defaults in BC or Ontario.
Choose a default Service Location and save the settings.