How do I redirect to a URL

It looks like this creates a redirect loop since the redirected request has the same hostname as your if statement. Try:

when HTTP_REQUEST {
 if {[string tolower [HTTP::host]] equals "webdrop.xxx.com" and not ([string tolower [HTTP::uri]] equals "/cgi-bin/wf.html")} {
            HTTP::redirect "https://webdrop.xxx.com/cgi-bin/wf.html"
 }
}