Marco Russo and Alberto Ferrari give us an option:
When Microsoft announced that DAX User-defined functions (UDFs) are generally available (GA), another new feature was also announced: it is now possible to define optional parameters in a function and assign them default values.
A parameter is optional when the caller can leave it out. In that case, the function still needs a value to work with, so it falls back to a default. DAX provides that default through an expression written directly in the function signature, next to the parameter it belongs to. This is the mechanism we describe in this article.
Read on to see how it works.