How can spawn a object in the center of screen?
For pixel-perfect precision, you need to convert your screen space coordinates to world coordinates while deciding on the object's distance from the viewing frustum's near clipping plane. It'll help to get your head around how the camera's view matrix works, which contains more or less everything you need to know.
For less accuracy you can just use the camera's location and move your spawn point along its out vector.
I went to look at my own examples but it turns out all I did was just that, spawn the new object at the camera's location and apply some force aligned with the view. Try googling "xna screen space to world space".