Custom Sign up - How to make public profile default

I had a custom sign up (Multistate box in a light box) coded into my site.
Right now all the members are signed up automatically as Private profiles
I need them to be Public so I can more quickly and effectively add them to groups.

I can’t see an easy way of adding a “Join the community” checkbox to this to make selected by default, as would be in a normal sign up form
Is it a case of needing to add even more code?
I cant see the command in the directory


Did you already try making member profiles public by default with this guide ?

Yes, I’ve read that. However it does not apply as as I’ve said, its a custom sign up process on a lightbox. That slider in the settings is already toggle on, but its irrelevant as there’s no check box in the sign up lightbox. I can add a checkbox, but i cant find a way to link it to the “back end”