Guide

1. Go to Setup in the Salesforce navigation bar.

2. Go to the Object Manager.

3. Locate the object that contains the lookup field you want to filter. Click on the object name. (Eg. Cases.)

4. Click on Fields & Relationships.

5. Hit New.

6. Find the lookup field you want to add a filter to and click on its name.

7. Hit Next.

8. Select the object your Lookup Field is related to and hit Next.

9. Provide an appropriate label

10. Hit Next.

11. Keep the default options as it is and hit Next on more time.

12. Keep the default value the same again and hit Next one last time.

13. Hit Save.

14. Hit Save.