Xero OAuth2

gistfile1.txt

public void Test()
        {
            // log in to the users xero account
            // send this call using browser: https://login.xero.com/identity/connect/authorize?response_type=code&client_id=E****************E&redirect_uri=https%3A%2F%2F**************************%2Fapi%2FAuthorize&scope=openid profile email offline_access accounting.transactions&state=123
            // it should redirect to the function app which will send back the code
            // the code only works once
            HttpClient client = new HttpClient();
            var clientId = "*******E";
            var secret = "Q***********q";
            client.SetToken("Basic", Base64Encode($"{clientId}:{secret}"));
            var values = new Dictionary<string, string>
            {
                { "grant_type", "authorization_code" },
                { "authorization", "6*********************************5" }, // paste code here
                { "redirect_uri", "https://**************/api/Authorize" }
            };

            var content = new FormUrlEncodedContent(values);
            var response = client.PostAsync("https://identity.xero.com/connect/token", content).Result;

            var responseString =  response.Content.ReadAsStringAsync().Result;
        }

From where i can get authorization code

Follow instructions in the commented lines