Chris Webb explains how to let end users work with Dynamic M parameters:
Dynamic M parameters are incredibly useful in Power BI when using DirectQuery mode because they allow you to pass values from your report direct to your data source query, bypassing Power BI’s own SQL (or whatever query language your source uses) generation layer. However it isn’t obvious how to pass any value you want: dynamic M parameters must be bound to slicers or filters, and they must be bound to tables in your dataset, which means that at first glance it seems like you can only pass values that are already present somewhere in your dataset (for example in a dimension table) to a dynamic M parameter. This isn’t true though: there is a way to allow passing of any value your users enter to a dynamic M parameter and in this post I’ll show you how.
Click through for an explanation of why you might want to do this, as well as a demonstration of how you can.