The MB Relationships extension automatically creates 2 meta boxes for 2 sides of each relationship with these IDs:
So you might need to use these IDs to include into the front-end forms to let users select a post/user to make a connection.
To display connected items in the front end, you need to code a little bit. The code is provided in the docs. As you’re using Beaver Builder, I’d suggest creating a shortcode to display related items, then insert the shortcode into a text module.