Generating an application access token using eBay RESTful APIs Error 401 grant type in request is not supported by authorization server

Question

https://developer.ebay.com/api-docs/static/oauth-client-credentials-grant.html

I am trying to generate an application access token in bubble.io but am getting hit with error 401 grant type in request is not supported by authorization server.

When I preview my page, chrome developer tools displays the following error:

{"error":"unsupported_grant_type","error_description":"grant type in request is not supported by the authorization server"}