How to prepend string to existing URI

Let me know if this helps. Set the following iRule. You can also do this using policies as well.

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/local-traffic-policies-getting-started-12-1-0/1.html

  when HTTP_REQUEST {
   if { [HTTP::uri] eq "/"}{
      HTTP::redirect [string map {/param1&param2 /param3&param1&param2}[HTTP::uri]]
        }
      }