Problem:
Include parent custom field value in form notification

Solution:
You can use the Toolset form hooks to add parent field using custom place holder:
- cred_body_notification_codes
- cred_subject_notification_codes

You can find the proposed solution in this case with the following reply:
=> https://toolset.com/forums/topic/include-parent-custom-field-value-in-form-notification/#post-1966263

Relevant Documentation:
- https://toolset.com/documentation/programmer-reference/forms/how-to-use-custom-placeholders-in-cred-notifications/