Hi,
Thanks for reaching out. To retrieve the monthly search volume data for specific keyword via API, you need to pass the search query (keyword) and the RequestType must be set to STATS to request the historical stats of a specific keyword. Please refer to this guide for more details and code samples. You could retrieve the historical statistics for keywords (Monthly Search Volume, etc.) to help you decide whether to use these keyword ideas or not. You might find this part of the guide useful on how to get monthly search volume. Let me know if you have any concern.
Thanks,
Nirmita Patel Google Ads Team