URL persitance iRule using UIE

I am tring to write an iRule to hash the hostname with CRC and persist all connetions for a specific URL hostname to one server, I am unable to add the uie add command in the HTTP_REQUEST context.

Is there another way to do this?

when HTTP_REQUEST {
 set FQDN hash persistance

set host [string tolower [HTTP::host]]
set hash [crc32 "$host"]
set mod [expr $hash % 6]

log local0. "Matching $mod in datagroup"

    if {[persist lookup uie $mod] equals ""} {

        persist uie [$mod]
    } else {
        log local0. "UIE already exists for $mod" 


    }

log local0. "Matching [IP::client_addr] on $mod"
}