The use of the ellipses (...) on a button is fairly well used to denote that the function requires more user input.
From the Microsoft User Experience Guidelines:
While menu commands are used for immediate actions, more information might be needed to perform the action. Indicate a command that needs additional information (including a confirmation) by adding an ellipsis at the end of the label.
Proper use of ellipses is important to indicate that users can make further choices before performing the action, or even cancel the action entirely. The visual cue offered by an ellipsis allows users to explore your software without fear.
This doesn't mean you should use an ellipsis whenever an action displays another window—only when additional information is required to perform the action.
In case of ambiguity (for example, the command label lacks a verb), decide based on the most likely user action. If simply viewing the window is a common action, don't use an ellipsis.
Use an ellipsis in the name of a menu item or button that produces a dialog. The ellipsis (…) indicates that the user must take further action to complete the task. The dialog title should be the same as the menu command or button label (except for the ellipsis) used to invoke it. To learn more about using an ellipsis, see Using the Ellipsis