How to persist on value in the URI

What you are referring to is what we call Universal Persistence.

To do this you need to use a rule to extract the portion of the uri you want to persist on and then execute the persist uie command with that value.

Here is an example that persists on the uri query string's "srvid" value:

 
 rule uri_persist { 
    when HTTP_REQUEST { 
       set srvid [findstr [URI::query [HTTP::uri]] "srvid=" 6 ";"] 
       if { $srvid ne "" } { 
          persist uie $srvid 
       } 
    } 
 }