May release of Power BI Desktop adds the capability of adding new calculated measures to a report when you are connected live to SSAS tabular. This is great because before this release, once you connected live to SSAS you could not make any changes to the model. Official announcement
Ideally the SSAS database has all the measures you need but now you have the capability to add new ones if you need to.
You can control the folder (table/measure group) under which the new measure shows up by using the “Home Table” option from the Modeling tab. I really like this feature as you can create copies of the same calculation and send them to different folders for ease of use.
Other SSAS related features
There is a request open for the same feature to be added for SSAS Multidimensional. Vote here:
There is another request for adding calculated columns to a report connect to SSAS Tabular live. I recently ran into a situation where I wanted to create a custom column for sorting since it was not provided in the SSAS database and I did not have permission to modify the SSAS database.