oAuth2 - Authenticate with 3rd party App

We like to develop an app for delegating authorization to other sites with owncloud oAuth2 (https://doc.owncloud.com/server/admin_manual/configuration/server/security/oauth2.html) . We have configured it perfectly in our server and owncloud desktop app is currently authenticating with oAuth2 and syncing with local files perfectly. We like to authenticate out the app with the owncloud oAuth2 app. Our site is like this-


And my created request is like this-

I can assure you, the client_id is generated with admin panel, so it is good to go. Can anyone please give me any suggestion about what I need to do to be authenticated from our developed app?

@DeepDiver1975 Can you have a look at this request?