Add checkbox to the SiteOrigin Contact Form widget

Hey guys,

thanks for your awesome themes and plugins. Really an evolution.

But as always there are some questions left. I am working for a client and want to use the SiteOrigin Contact Form widget as a kind of Newsletter function with some additional questions.
In germany we need a special approval with a checkbox and a short disclaimer text, for a newsletter. Any idea how i can add a checkbox and a short disclaimer text to the contact form widget? I am using your North theme.

Thanks in advance.