Adding a URI using IRULE

Try this:

when HTTP_REQUEST {
if { [HTTP::uri] equals "/"}
 {
    HTTP::redirect "https://[HTTP::host]/prl-ws//"
 }
}

Do you want to redirect (HTTP response 302 or 307 with new URL location header) or forward with new URI.

If you want to redirect use this code (Never insert the Host header in the redirect if the protocol and the host are the same than the client request):

when HTTP_REQUEST {
     don't evaluate the uri but the path (without query string) 
    if { [HTTP::host] equals "restricted-list" && [HTTP::path] equals "/"} {
         Change only the path part, keep the query string
         Use 307 instead of 302 (default redirect command) to force the client to post data if the first request was a POST.
        HTTP::respond 307 Location "/prl-ws/[HTTP::uri]"
    }
}

If you want to forward and change the URI in serverside request

when HTTP_REQUEST {
     don't evaluate the uri but the path (without query string) 
    if { [HTTP::host] equals "restricted-list" && [HTTP::path] equals "/"} {
         Change only the path part, keep the query string
        HTTP::path "/prl-ws//"
    }
}