Attention! Game PAYDAY 2 should already be installed before the activation of the key. The key gives only additional content: Pen Melee Weapon DLC and does not give the game itself!
This is the code you'll need to get your Pen Melee Weapon in PAYDAY 2.

Product Activation on Steam
To activate your digital key into Steam, follow the instructions below:
1. Go to the "Games" menu.
2. Select "Activate in Steam ..."
3. Follow the on-screen instructions to complete the activation of the game on Steam.