How to make Upsert-Mode functioning in absence record API?

Summary:

Our integration demands if an absence entry is already there for the combination of PersonNumber|AbsenceType|StartDate then incoming data should update the existing entry else create a new absence entry. I see that Upsert-Mode helps us with the above. However when I try to update the existing entry by passing the below with Upsert-Mode set to true in header.

{{InstanceURL}}/hcmRestApi/resources/11.13.18.05/absences

{
"personNumber": "123",
"absenceType": "Non-FMLA Medical",
"employer": "ABC LE",
"absenceReason": "Illness, injury",
"startDate": "2025-03-19",
"startTime": "00:00",
"endDate": "2025-03-20",
"startDateDuration": 7.5,
"endDateDuration": 7.5,
"absenceStatusCd": "SUBMITTED"
}

System shows the following error which is expected if system tries to create new entry instead of Merge.