We are developing a SharePoint 2013 Provider Hosted Add-In that calls an API in Azure. The API an App Service that has Azure AD Authentication and OAuth implicit grant type. Our wish is to authenticate the current user and do it on the client side, not use app permissions and run from code behind.
The catch is that a provider hosted add-in must have some url query strings like SPHostUrl. We cannot set the whole url in the App Registration Redirect Url, since SPHostUrl differs from site to site. When a user has logged in successfully in Azure AD, she/he will be redirected to the main page of Provider Hosted Add-In and loose SharePoint Context.