Hi @coachadnyc,
Welcome to the Auth0 Community!
In your request, you must use the application/json content-type header.
Navigate to your Auth0 Dashboard > Applications > APIs > Auth0 Management API and click on the Test Tab.
On there, click the drop-down menu for your Regular Web App, and copy the code snippet for C#. See below.
This should be the quickest way to get the correct working code snippet for getting Management API Access tokens for Production. I have tested this and can confirm that it works.
Please let me know how this goes for you.
Thank you.