JavaScript apps for the Windows Store do not have access to the built-in popup that looks or behaves like a Microsoft account login dialog box.
Windows.UI.Popups.MessageBox has a simple message box, but it does not have the ability to add additional controls, etc.
For your experience, you are looking for a modal user interface - to build, that you can build it relatively simply on your own (just add the position: absolute element to the body and make it fill the screen - the rest is that the elements / position of the children are under your complete control), or as Mayur suggested, use one of the many helper libraries that are there.
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