Here's your code:
Response.Redirect("urlToKecloakRealm",false);
string url = Request.Url.AbsoluteUri;
Did you notice that you said you want to "catch the
response url, but this is not happening"
However, the code is showing:
Request.Url.AbsoluteUri;
You've got Request there.
Are you simply referring to the wrong object?
Or did you state it incorrectly?
Just checking on that. Maybe this will help. :)