You have to understand the distinction between client and server.
The AJAX/DWF/Fusion viewer is the client.
Your scripts that use the Web API run on the server-side.
Digitizing geometries is a client-side function.
As a result, you can't really have client-side functionality (digitizing) on a server-side script.
What you can do however, is have your asp.net script Response.Write() out the required javascript code to digitize the geometry (ie. Response.Write() that javascript code)
Hope that helps.
- Jackie