The URI (uniform resource locator) is something you will get from the social media provider. Below is an example of how the URI is defined and configured for Facebook as an example - https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow. The purpose of this configuration is to trust one or more web-sites that you want users to be re-directed upon login/logout using social media applications like Facebook. Check with each of your social media providers for the URI that you can enter on the Centrify side for the redirect.
Hope this helps.