How to add missing Content-Length header to an HTTP POST request?

Can you post what you tried? Something like this perhaps? (to be applied on your layered vip) no need to call HTTP::release here, it's done implicitly. I'd make sure that posts hitting this vip are small, otherwise you'll want to exercise more care than I example here.

when HTTP_REQUEST {
  if { [HTTP::method] eq "POST" } {
    HTTP::collect
    set ispost 1
  }
}
when HTTP_REQUEST_DATA {
  if { [info exists ispost] } {
    HTTP::header insert Content-Length [HTTP::payload length]
  }
}