The traditional method for making a hole in a TCad object is to subtract a cylinder (or perhaps a more specialized shape) from the object. Correctly orienting the cylinder is likely to be difficult if the hole must be normal to a curved surface, such as the one shown in the attached screenshots. Here's a trick that often works:
In Options|Display, enable "Draw form-building edges" -- whereupon objects in the drawing will be displayed as sets of rectangular meshes. Zoom in very close to the particular rectangle that encloses the point where you want to make the hole; enable "Workplane by facet"; click inside the mesh rectangle. Don't be concerned if there is no visual indication that you've designated this particular spot, but what happens is that the new Workplane is tangent to the surface of the target object at the spot defined by the mesh rectangle. Draw a circle there and extrude it. There's the cylinder you can use to make the hole.
EDIT: If your version supports "Auto Workplane by Face," use that instead of "Workplane by Facet." You'll probably have to convert the object to a TC Surface to make AWBF functional on a curved surface. For insurance, place a copy on a hidden layer before converting. END EDIT
The attached "Form-building Edges.jpg" shows several green circles drawn on a complex curved surface with its form-building edges made visible. "Normals to a Surface.jpg" shows extrusions made from those circles.