How to keep the internal URL in browser address bar and point to external website

Try something like this example:

when HTTP_REQUEST {
if { [HTTP::host] contains "abcd.com" } {
HTTP::host [string map {abcd.com xyz.com} [HTTP::host]] 
}
}

when HTTP_RESPONSE {
if { [HTTP::header values Location] contains "xyz.com" } {
HTTP::header replace Location [string map {xyz.com abcd.com} [HTTP::header value Location]]
}
}