You need to send the user you wish to auth to that URL. You shouldn’t be using fetch, or cURL, or anything like, you need to send the user to that link.
Once the user is on that Twitch auth page, they can choose to accept or deny the authorization, at which point they’ll be redirected to your server. From that point on your server can continue the process programmatically, exchanging the code for an Access Token and Refresh Token, and using the refresh token to get new tokens each time they expire, but that first step can not be done programmatically as it requires the user to go to that link and explicitly consent to the scopes you’re asking for.