In Office 365, regardless if you are using ADFS or have configured a custom login screen (using Azure AD Free), you will still be presented with the standard Login Screen when for example accessing the Portal (portal.office.com) or SharePoint Online (tenant.sharepoint.com).
If using federation for single sign-on, entering the email address is enough to be redirected to your ADFS server. When using Synchronized Identities or Cloud Identities with a customized sign-in screen the process is similar, you will be redirected to your organizations login screen when you type the user name.
But what if we want to show our organizations branded login screen or ADFS login before the username is typed? Exchange Online has had this possibility since long, by using the URL https://outlook.com/tenant.onmicrosoft.com, but there is another solution that work for all Office 365 services.
Let’s start by looking on the URL you get redirected to when logging on to SharePoint Online:
There are two important parameters in this URL:
whr=tenant.onmicrosoft.com: This controls which login screen to use
wreply=https%3A%2F%2Ftenant%2Esharepoint%2Ecom%2F: The URL to redirect to after a successful logon (URL must be percent-encoded).
By replacing the whr and wreply parameters you can build your own links and deploy them in your organization, for example by setting the home page to point to your intranet in SharePoint Online. Regardless the method your organization are using for authentication, the users will now always be presented to the correct login page!