Position relative to the camera?

I’ve got a separate camera for GUI elements so that when I walk around they aren’t going through walls. My problem is that I want to track an object in the scene by placing a marker on it but I don’t actually want a physical marker, so I need it to be on my GUI camera. So how would I get the world position of the object and convert that to a point on my GUI camera so that it is always on top of the object? I tried using WorldToViewportPoint but that didnt’t work at all. Any ideas?

Marker, world position, swap to screen position in GUI cam.