taking value and append to uri while masking?

I was able to get it to work using the following syntax:

when HTTP_REQUEST { Check for a non-null length host header if {[string match {[a-zA-Z]} [HTTP::host]]}{

   Use string commands to parse the subdomain
  set subdomain [string range [HTTP::host] 0 [expr {[string first . [HTTP::host]] -1}]]

   Use the getfield command to parse the subdomain
  set subdomain [getfield [HTTP::host] . 1]

   Use scan to parse the subdomain
  scan [HTTP::host] {%[^.]} subdomain

    HTTP::header replace Host "www.test123.com"
    HTTP::uri /$subdomain[HTTP::uri]

} }