@Ajin vs , Thank you for reaching out. I do see that all the settings for the app registration are as per the document. But based on this, it would be hard to help you fix this issue. The error on the screenshot shows 403 - Forbidden, which I feel might be coming up if there the contents in the Body is not correct and hence not getting accepted. I just tried to create an onlineMeeting using the same API and Postman and I got it working.
Do try it with Postman once to make sure you are it works fine in Postman before testing the same with your application.
Hope this helps.
Do let us know if this helps and if there are any more queries around this, please do let us know so that we can help you further. Also, please do not forget to accept the response as Answer; if the above response helped in answering your query.