Putting a fixation cross along with my other visual stimuli which move on the screen

Hi everyone. I have a visual task in which rectangles move on the screen from side to side. Now I need to put a fixation cross or fixation point at the middle of the screen as a point where subjects need to look at instead of following the moving rectangles. How can I put a fixation cross without affecting the moving object?

What goes wrong when you try it?