Unable to post to an external API endpoint from an App Service website in Azure

Question

I am doing an HttpPost to the FedEx tracking API endpoint from the backend of a web app that I am hosting in Azure.

The app works fine on my development machine and can post to the endpoint just fine. But after deploying it to my App service it is no longer able to call the FedEx api to get the required oauth token.

 AccessTokenInfo responseObj = new();
 using (var client = new HttpClient()) {
      client.BaseAddress = new Uri(_fedexConfig.BaseUri);
      client.DefaultRequestHeaders.Accept.Add(new  
            MediaTypeWithQualityHeaderValue("application/json"));
            var response = new HttpResponseMessage();
            var allIputParams = new List<KeyValuePair<string, string>>() {
                 new KeyValuePair<string, string>("grant_type","client_credentials"),
                 new KeyValuePair<string, string>("client_id",_fedexConfig.ClientId),
                 new KeyValuePair<string, string>("client_secret", _fedexConfig.ClientSecret)
                 };
            HttpContent requestParams = new FormUrlEncodedContent(allIputParams);
            response = await client.PostAsync(_fedexConfig.TokenUri,requestParams).ConfigureAwait(false);
       }

When the call is made the response only returns a 403 error "forbidden".

Is there some additional settings that the AppService needs to have to communicate with an api endpoint outside the Azure environment?