1. Create a Quick Action on Case object with the name Close Case and Action Type as “Update a Record”. In the Predefined Field Values, set Status field value to Closed.

 

2. Add Internal Comments field in the layout.

 

 

3. Add the Quick Action to the Case Page Layout. Don’t add the standard Close Case. Add the custom Quick Action created in Step 1.

 

4. In the Lightning Record Page, add Chatter Publisher or Chatter Standard Component.

5. Now, create a Macro. When you click Add Instruction, the Close Case quick action will be highlighted to select. Select Internal Comments Field and add text to it. 

Note:

Internal Comments entered will be stored as Case Comment in Salesforce.