Getting "error": "unsupported_grant_type" when trying to get a JWT by calling an OWIN OAuth secured Web Api via Postman

The response is a bit late - but in case anyone has the issue in the future...

From the screenshot above - it seems that you are adding the url data (username, password, grant_type) to the header and not to the body element.

Clicking on the body tab, and then select "x-www-form-urlencoded" radio button, there should be a key-value list below that where you can enter the request data

With Postman, select Body tab and choose the raw option and type the following:

grant_type=password&username=yourusername&password=yourpassword

1. Note the URL: localhost:55828/token (not localhost:55828/API/token)
2. Note the request data. Its not in json format, its just plain data without double quotes. [email protected]&password=Test123$&grant_type=password
3. Note the content type. Content-Type: 'application/x-www-form-urlencoded' (not Content-Type: 'application/json')

4. When you use JavaScript to make post request, you may use following:

   $http.post("localhost:55828/token", 
     "userName=" + encodeURIComponent(email) +
       "&password=" + encodeURIComponent(password) +
       "&grant_type=password",
     {headers: { 'Content-Type': 'application/x-www-form-urlencoded' }}
   ).success(function (data) {//...
   

See screenshots below from Postman: