Hi,
The API that you are looking for is Reservation Summaries. It applies to any type of reservation and basically will give you the utilization in % via avgUtilizationPercentage property. Keep in mind that if the end date for your request is the current most likely the utilization will not be 100 %. This is due to delay in data. This API is the same one when you go to Azure Portal -> Reservations -> click on specific reservation and you will see utilization over time chart.
Edit: Also check Reservations Details API.
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