Adding this post in the Tableau Prep forum with specific search juice (keywords: cube, SSAS, Analysis Services, MDX, DAX, etc).
As of now (2018.1.2), Tableau Prep does not have a built-in connector to Microsoft Analysis Services cubes (either dimensional or tabular).
There is already an Idea post suggesting this be added: https://community.tableau.com/ideas/8727
If you've stumbled upon this post searching for such SSAS connectivity, be sure to log in, go to that Idea post, and +1 upvote it to show the value of working on this connector.
Thank you for sharing and encouraging more votes.