There is currently no default source for Google My Business links.
Indeed, Google feeds exactly the same referrer (parameter that determines the source of the visit) for traffic coming from the classic Google search engine, as for traffic coming from My Business links.
To measure the number of visitors coming from your Google My Business link, we suggest that you enter your website URL with a campaign parameter (whether natural or marketing).
Natural source: http://www.mysite.com/?origin=googlemybusiness (requires the creation of a custom natural source based on origin=googlemybusiness)
Marketing source: http://www.mysite.com/?at_medium=googlemybusiness&at_campaign=googlemybusiness (requires the creation of a custom marketing source)