Is it possible to get the system case Id by the custom field?
You can get the caseSystemId using the caseId. It is not possible to fetch the caseSystemId using the custome fields.
User the below API call to acheive the caseSystemID -
If using postman collection v1.3 - SEQ-3: Get the system ID of a case
if using postman collection v1.5 - SEQ-case-retrieve: Get the system ID of a case
Can you please elaborate a bit more on your concern, so that we may understand what exactly are you trying to acheive?