In the first level of the game "Point Insertion" after your run in with Barney you will come across a Civil Protection officer standing guard at a trashcan. The CP will knock the can to the ground asking you to pick it up and throw it away.
To earn the Submissive achievement pick up the can by looking at it and hitting E, aiming at the trashcan and clicking your LMB to throw it into the trash.
Congrats! You're now a sheep and have earned an achievement!
Alternatively, you can throw the can into the CP's face to earn: