All of our Design / How-To guide are located here but unfortunately, we don't have anything specific regarding what you are asking for. Configuring ISE to redirect to an external server requires the use of Advanced Attribute Settings in an Authorization Profile. In it you would specify the external server to be used. For example:
cisco-av-pair = url-redirect-acl=PreAuth_ACL_Containing_External_Server
cisco-av-pair = url-redirect=https://external-server
Regards,
-Tim