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I was able to get the function to work, using the following code:

Reference: https://developers.ringcentral.com/guide/authentication/jwt-flow

procedure TfrmDialRC.GetTokenRCJasonWebToken; /

var

htp: TIdHTTP;

prms_stm: TStringStream;

s: String;

begin

// This works!

try

htp := htpTkn;

if htp.Connected then

htp.Request.Connection := 'close';

htp.Request.Clear;

s := 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=' + GV.RingCentral.JWT;

prms_stm := TStringStream.Create(s);

prms_stm.Position := 0;

htp.Request.Accept := 'application/json';

htp.Request.ContentType := 'application/x-www-form-urlencoded';

htp.Request.Username := GV.RingCentral.ClientID;

htp.Request.Password := GV.RingCentral.ClientSecret;

htp.Request.BasicAuthentication := True;

htp.Post( 'https://platform.ringcentral.com/restapi/oauth/token', prms_stm);

finally

prms_stm.Free;

htp.Request.Connection := 'close';

end;

end;

I recommend you to submit a dev support ticket and someone can have a look at your app settings and help you out with postman API call too.

The function you implemented to use the JWT token is totally incorrect. Why do you still specify the user name and password? And I don't know how the Request.BasicAuthentication := True sets the Authorization header.

I don't have sample code in Python. But I have native code in PHP showing how to use the JWT token to exchange for access tokens.

private function authenticate(){
      $url = $_ENV["RC_SERVER_URL"] . "/restapi/oauth/token";
      $basic = $_ENV["RC_CLIENT_ID"] .":". $_ENV["RC_CLIENT_SECRET"];
      $headers = array (
              'Content-Type: application/x-www-form-urlencoded; charset=UTF-8',
              'Accept: application/json',
              'Authorization: Basic '.base64_encode($basic)
            );
      $body = http_build_query(array (
              'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
              'assertion' => $_ENV["JWT_TOKEN"]
            ));
      try {
        $ch = curl_init($url);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_POST, TRUE);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($ch, CURLOPT_TIMEOUT, 600);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $body);

        $strResponse = curl_exec($ch);
        $curlErrno = curl_errno($ch);
        if ($curlErrno) {
          throw new Exception($curlErrno);
        } else {
          $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
          curl_close($ch);
          if ($httpCode == 200) {
            $date = new DateTime();
            $jsonObj = json_decode($strResponse);
            $tokensObj = array(
              "tokens" => $jsonObj,
              "timestamp" => $date->getTimestamp()
            );
            // Logged in successfully => Save tokens in session or write to file/database for reuse
            $_SESSION['sessionAccessToken'] =  json_encode($tokensObj, JSON_PRETTY_PRINT);
            return;
          }else{
            throw new Exception($strResponse);
          }
        }
      } catch (Exception $e) {
        throw $e;
      }
    }

You can get the entire project from here and convert the essential code to Python.