Technologies are described herein for incorporating custom user interface (UI) forms into a personal information manager (PIM) application using a declarative form markup specification. A native PIM form specification is generated from a file containing the declarative form markup describing the custom UI form. When a request to display the custom UI form is received, a PIM native form renderer invokes a forms runtime engine that loads the original declarative form markup to process data bindings and event subscriptions contained therein. The custom UI form is then rendered to the display by the PIM native form renderer in conjunction with the forms runtime engine.