The primary purpose of the Circle tool is to draw circles. The radius information is provided only for reference.
It is possible to use the standard ASP controls to enter the desired radius, set it to the Map.mapToolArgument property (AspMap Cleimnt-side API and the use for example the Point tool to specify the center point. The Point tool will submit the point and the radius to the PointTool event handler on the client-side.
There is also the client-side event for the Point tool (see the documentation).