MediaWiki:Api-help-authmanagerhelper-returnurl
From Unofficial Zero Manual
Return URL for third-party authentication flows, must be absolute. Either this or $1continue is required.
Upon receiving a REDIRECT response, you will typically open a browser or web view to the specified redirecttarget URL for a third-party authentication flow. When that completes, the third party will send the browser or web view to this URL. You should extract any query or POST parameters from the URL and pass them as a $1continue request to this API module.
