You’re getting a web page because you need to actually send the user you need an OAuth token from to that URL. Once their on Twitch they’ll see a confirmation box asking if they wish to grant your app whichever scopes you’ve requested, and then they’ll get redirected to your redirect URL.
Alternatively if you don’t require any user scopes, and are doing this on your server, you can use the Client Credentials flow to generate an App Access Token, which doesn’t require user interaction.