To set up a case rule, please see the steps below.
1. Navigate to Setup > Support > Case Rules > New.
2. Under Case Fields, select Incoming Message.
3. In the Name field, enter a name for your case rule
4. In the Criteria field, select 'contains' and in the Incoming Message field, enter the keywords
5. Click Save
Note: The Incoming Message field pertains to the text body of the email. If the user would like to pick up some keywords from the Subject line, select Title as the case field when creating a case rule. Also, please note that the Incoming Message case field works only for inbound emails and for online case forms. For the case field Title, it works for inbound emails and when creating cases in the user interface.