I'm not asked to accept permisions after redirect using OAuth service


I’m creating a SmartApp that will use an internal third party OAuth service to ask the user to introduce their credentials.
Everything works fine in the first configuration process. However, when I need to redirect the user to https://api.smartthings.com/oauth/callback from the OAuth service to receive OAUTH_CALLBACK, so the user can select now what devices he wants to link, nothing happens. After sending redirect to that URL from the OAuth service, I’m redirected to the menu where I need to tap to sign up to access the OAuth service.

am I missing something? Thank you!