Hi Metin, if your solid is planar on that side, you can just do a regular extrusion.
Also if you want a hole all the way through you can do a boolean with a 2D closed curve to cut a hole all the way through.
If you want something that's more like an indentation not going all the way through on a curved surface, then that's a little bit more complicated. Often times the easiest way to do that is to cut the curved surface using Trim, then extrude the surface.
Check out these links for some description on this:
One thing to note is that Trim has projection built into it, so you don't need to project first and then trim, just trim directly with a 2D curve and it projects built into it.
This kind if indentation effect is something that I'm going to try to build more directly into the booleans in the future. Right now you can do a boolean with a 2D curve which a hole all the way through, I want to add a "depth limited" cut as an additional option.
