str = "accountRequest=" _
& Server.URLEncode("<NewUser>" & vbCrLf & "Hello" & vbTab & "World" & vbCrLf & "</NewUser>" )
ASP code is, after all, running on a Windows platform, so you really should use CrLf as a line separator, not Lf alone.
And you NEED Server.URLEncode() to ensure that many characters aren't misinterpreted. Dunno whether that applies to vbTab, but it's worth a shot.