I appreciate you showing interest about this feature, Pam. Right now, we can only add "CM" manually in the credit memo number box. Let me walk you through about this workaround.
Currently, the option to automatically add "CM" to every credit number is unavailable. However, you can manually add it when creating the credit memo.
Once added manually, the system will then automatically include "CM" each time you create a new credit memo for your customers.

Furthermore, refer to this article for detailed steps in creating a credit memo in QBO.
Additionally, refer to this article about recording refunds to your customers if they have overpaid their payment.
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If you have any other concerns related to QuickBooks, please feel free to reply to this thread, Pam. We're here to provide solutions to your needs.