Hi,
If I understand well, you want your user to automatically subscribe in one list and then have a choice to subscribe to another one.
If so, you can do it without a custom field, in the widget you'll have to select your list in the field Automatically subscribe to and then the other list in the field Displayed lists, like that:
If you want to keep up with the custom field, can you tell me which filter and action you've set up?