How to make area appoint mechanism?

Unity Engine
1

Please see this video

In this video, when user click skill key, territory image(circle) appear and user can move that image with mouse movement and appoint area with left mouse button.

I want to make spell that appoint area(territory) before launched.

What mechanism in unity can realize that function(territory appointment)?

2

You can use a raycast from the camera to find the position on the landscape under the mouse (see this thread to find out how to do this). Once you have that point, you can use a projector to overlay a texture on the surface.