I tried to use a MapButton but Map crashed after clicking on it. MapButton allows the user to click or draw a rectangle in the drawing area and returns the coordinates for further processing. See developer sample #30.
As alternative one can use the following call:
Autodesk.Map.IM.Graphic.Polygon poly = this.Application.Documents.Active.Map.GetRectangle("first point:", "second point:");
further details Map API Doc: MapLogic.GetRectangle Method
Map 2103, SP2