How to create or edit MDX Query text in Reporting Services dataset definition

A frequently-asked question in my SSRS and/or SSAS classes goes something like this: "How do I enter my own MDX query in an SSRS report rather than use the query builder?" While the GUI doesn't make it obvious, the answer is to use the Design Mode toggle button in the Query Designer:

WARNING: if you hand-edit the query statement, and then toggle the Query Designer button, your query text will be lost. Be careful! (Happily, the UI does provide a warning dialog box.) Update: for some reason, the graphic is being cut off, hiding the pointer to the toolbar button I'm depicting. Please use this link to see the full picture, until I can re-edit the post