iRule to detect browser and display warning message

OK so assuming my syntax is correct I'd have something like the iRule below. How can I capture the address entered to the user so I can redirect him to that address once he click "Continue" on the warning page?

when HTTP_REQUEST {
    if { not ( [HTTP::cookie exists WRONGBROWSER] ) AND [class match [string tolower[HTTP::header "User-Agent"]] contains "firefox"]} {
        set expires [clock format [clock seconds] -format "%A, %d-%b-%Y 20:00:00"]
        HTTP::respond 200 content "
            HTML
            click here to continue (with a link to https://whatever_he-was_trying_access
            /HTML
        " "Set-Cookie" "WRONGBROWSER=1; path=/; expires=$expires"
    }
}