Manual:$wgRedirectOnLogin

Details
$wgRedirectOnLogin uses the   URL parameter to redirect users to a specific page when they log in.

Specify the name of a page in the wiki where they should be sent automatically when they log in, for example: 'Main Page', or 'Special:MyPage'.

Setting this option to null will make the user be redirected back to the page they came from.

Examples
This will redirect users to 'Special:MyAuth'.