$wgOAuth2Client['client']['id'] = ''; // The client ID assigned to you by the provider
$wgOAuth2Client['client']['secret'] = ''; // The client secret assigned to you by the provider
$wgOAuth2Client['configuration']['authorize_endpoint'] = ''; // Authorization URL
$wgOAuth2Client['configuration']['access_token_endpoint'] = ''; // Token URL
$wgOAuth2Client['configuration']['api_endpoint'] = ''; // URL to fetch user JSON
Covers client_id and secret
The rest is in
https://id.twitch.tv/oauth2/.well-known/openid-configuration