Creating a custom unknown member in BISM Tabular

One nice feature that UDM / Multidimensional SSAS cubes have is that of being able to specify a custom name for the 'unknown' member - that is, the member automatically generated by the presence of foreign keys in a fact table that refer to members not present in the dimension table.

In Tabular, the default behavior is to assign a blank member. We can easily see this with an example: