I am trying to pick out a string of a decrypted JSON web token to set some paramters in it as variables that i can later use to refer in a custom HTTP header. So, the sample token i have is

Hi

Try something like this....

when HTTP_REQUEST {
  if {[HTTP::method] eq "POST"}{
    if {[HTTP::header "Content-Length"] ne "" && [HTTP::header "Content-Length"] <= 1048576}{
      set content_length [HTTP::header "Content-Length"]
    } else {
        set content_length 1048576
    }
    if { $content_length > 0} {
      HTTP::collect $content_length
    }
  }
}
when HTTP_REQUEST_DATA {
    set payload [HTTP::payload]
  set oid [findstr $payload "oid:" 4 " "]
  set tid [findstr $payload "tid:" 4 " "]
  HTTP::header insert OID $oid
  HTTP::header insert TID $tid
  log local0. "OID = $oid - TID = $tid"
}