HTTP Post to Odata service

I would like to create a HTML page to post data to a ODATA service (basically what Thomas Jung showed in this: https://www.youtube.com/watch?v=c41anxrDleg but without using Postman).

A simple HTML page with POST is not working. Is there any way to make it work with HTML, POST method?

As an alternative, I tried to use ajax like below:

But keep getting 400 (bad request) error.

I would appreciate any help with this.

Thanks