There is no specific code to get pro headshots, but there are a few things you can do to increase your chances.
First, make sure you have the latest version of the game installed. The headshot mechanics were changed in the latest patch, so if you're not up to date you won't be able to get them as easily.
Second, understand the mechanics of headshots. Headshots are now based on damage per bullet rather than chance. This means that weapons with high damage per bullet, like sniper rifles, are more likely to get headshots. Conversely, weapons with low damage per bullet, like pistols, are less likely to get headshots.
Third, practice your aim. This is probably the most important factor in getting headshots. The more you play, the better you'll get at aiming for the head.
Finally, use a weapon that you're comfortable with. If you're not comfortable with a certain weapon, you're not going to be able to aim as well with it. So find a weapon that you like and practice with it until you're confident you can get headshots with it.
