//  here I get the 3D position of mouse pointer
Vector3 target = camera.GetWorldPositionFromScreenSpaceCoordinates(new Vector3(e.X, e.Y, 0));
// now get a vector from the camera in direction of the mouse pointer
Vector3 dir = (target - camera.Position).Normalized();
// A ray from camera to the direction 
Ray ray = new Ray(camera.Position, dir);
// Now I create a plane that faces the same direction as my UI object (in my case I've created a Neosis Grid object with a button inside)
Plane plane = new Plane(button1.Position, button1.Direction);
float? result = ray.Intersects(plane);
if (result.HasValue)
{ 
 	// If my ray intersects the plane then I check if I'm hitting the button, first I get the exact position that my ray hit the plane
	Vector3 pointInWorld = camera.Position + dir * result.Value;
	// Now I convert this position to a local position relative to the button
	Vector3 relativePoint = Vector3.TransformPosition(pointInWorld, button1.Transform.Inverted());
	// Get the 3D button dimensions and calculate normalized values for X and Y (meaning a value that goes from 0 to 1). Actually they can be negatives, which means the mouse click happened either more to the left or more to the top (or both) than the button position.
	// The x value goes from left to right and y value from top to botton
	Vector3 buttonDimensions = button1.GetDimensions();
	float x = (relativePoint.X + buttonDimensions.X * 0.5f) / buttonDimensions.X;
	float y = 1f - ((relativePoint.Y + buttonDimensions.Y * 0.5f) / buttonDimensions.Y);
	// Now I get the actual 2D position in the button and trigger the MouseMove method in Neosis view object. It might result in negative values, meaning the click position is outside the button bounds. 
	Vector2 v = new Vector2(button1.Texture.Width * x, button1.Texture.Height * y);
	button1.MouseMove((int)v.X, (int)v.Y);
}