Generally your service will be available for more than one organization. When a user logs in, his Identity Provider must be determined. This is called IdP Discovery and is done by a Where-Are-You-From (WAYF) selection page.
You can use:
- WAYF page from SURFconext (= default)
- or create your own one.
Please have a look at the excellent Refeds Discovery Guide on how to implement the best sign in experience!