I am trying to display a dialog that allows the user to type a location, just like in the "add places" function of the Weather App on Windows 8.
Windows.UI.Popups namespace does not have an appropriate control. It has the MessageDialog, but I don't think it can be customised to include a textbox in it.
Would I need to use Windows.UI.XAML.Controls.Primitives.Popup control by any chance?
There is no out of box control beyond Popup to handle this style of UI, the Callisto library uses this control quite a bit so it has a lot of good examples of it's usage.
Edit: In fact now the Callisto library has the CustomDialog control to help you do exactly this.