CheckboxList field default value


Active Member
Is there a way to set the default value of an optional checkboxlist field?
I have a number of content options users can opt out of via the "profile" (=preferences) page after initial signup. However upon first sign-up, all these options should be active by default.
To keep my signup form short, I'm only asking for FNAME, LNAME, EMAIL. The optional fields (COUNTRY, CONTENT_PREF etc.) are then available to be edited through the profile page.

If content preferences are...

... when all ticked, the field would contain A, B, C.

So I tried to set "A, B, C" as default value for this field with the hope that this would always populate the field for each new record. However the field stays blank.
Any idea?