If you want to obtain an access token to call the Authorization Extension API then have in mind that the URL to obtain the token is different then the URL to call the API.
In particular, you’re performing a request to https://idigitalasset.us.webtask.io/adf6e2f2b84784b57522e3b19dfc9201/oauth/token and as indicated in (Call Your API Using the Client Credentials Flow) you need to call the token endpoint of the tenant in which the extension is installed.
In other words you should be getting a token from a URL similar to https://YOUR_AUTH0_DOMAIN/oauth/token; in your case your tenant seems to be idigitalasset@us so the token endpoint would be located at https://idigitalasset.auth0.com/oauth/token.
After having obtained the access token you then do need to call the API at an URL such as https://YOUR_TENANT.us.webtask.io/adf6e2f2b84784b57522e3b19dfc9201/api.