Will it work without searing it first? Yes.
Will it have as much flavor? No.
Searing does two things: Create flavor through the browning process and jump-start cooking. Searing does not "lock in juices". The mere sound of the sizzling that goes on is indication that juices are exuding and sizzling against the hot cooking surface.
Benefit of Searing for a Slow Cooker or Pressure Cooker recipe:
In the case of preparing meat in a slow cooker or in a pressure cooker, the purpose of first searing the meat is to brown the meat for better flavor.
If you have the time and don't mind having another pan to clean, then sear the meat before adding it to your slow cooker. If time does not permit then it will still work to put the meat in without first searing, the flavor of the finished dish will just be a bit more subtle and not as "meaty".