Create new Form Builder Meta Box
A form builder meta box is a normal post meta box and can be added with add_meta_box().
add_meta_box( $id, $title, $callback, $screen, $context, $priority, $callback_args );
WordPress Codex: https://codex.wordpress.org/Function_Reference/add_meta_box
Just make sure you use "buddyforms" as post type to make sure the meta box is only loaded in the BuddyForms edit screen.
In the above example we add a checkbox to the meta box.
You can use any kind of form element. As long as you make sure you add it to the "buddyforms_options" array all the rest like saving and validating will be done automatically. Just make suer you use the structure "buddyforms_options[YOUR OPTION]" in your form element name.