Adding Interactive Components to a Model

Greetings

I am trying to develop a model that will create contours from a smaller location defined by the user. I would like the process be set up so that when a user runs the model, they are prompted to draw an area of interst on the map which will be used as a clip tool to extract the area and then contour. How do I set up the interactive part? This will eventually be shared as a geoprocessing service.


Regards
Bruce Burwell
Sudi Aramco