A common way to do this is a two-shot practical illusion rather than heavy editing.

1. Photograph your hand in front of the monitor, with the frame wider than the screen so the hand extends beyond the monitor edges.
2. Put that photo on the computer as the desktop background.
3. Place a window, such as Notepad, so it overlaps the hand image where you want the effect.
4. Take a second photo, this time framed tightly so only the screen area is visible. A screenshot can also work for the screen content, though cursors may not appear.

Because the second photo only shows the monitor area, the hand in the wallpaper looks like it is physically in front of the open window.

Another possible method is an even simpler physical trick: print the original image, cut out the hand, and hold that print in front of the screen while photographing it. That creates a similar forced-perspective effect with no Photoshop at all.