I'm using 2 forms on the same page, below is the code.
1. Add Form (On Modal window) - [mb_frontend_form post_type="directory" post_fields="title,thumbnail,content" id="generic-info,company-info,location,additional-info,social"]
2. Edit Form (On Different Modal window) - [mb_frontend_form post_type="directory" post_id='.$id.' post_fields="title,thumbnail,content" id="generic-info,company-info,location,additional-info,social"]
where $id = $post->ID;
I'm following the shortcode and its attributes based on the documentation available on MB Builder page, please highlight if I missed/messed up anything here.