Add Theme support for CPT for Jetpack Publicize plugin

I have activated Jetpack Social which is a plugin from Wordpress that adds the publicize module to the publish box on a post. This works fine on a normal post but on a custom post type it does not work. From what I can tell one needs to add some PHP to the Child theme function.php file or create an MU plugin or snippet to add support for custom post types. However, I don't know how to do this and would like someone to do this for me please :)