Client (multipart_form-cohttp-lwt.Multipart_form

val multipart_form : 
  Multipart_form.multipart ->
  Cohttp.Header.t * Cohttp_lwt.Body.t

multipart_form v returns a well-formed CoHTTP header with a CoHTTP body which can be used by the client implementation of CoHTTP:

let headers, body = Multipart_form_cohttp.Client.multipart_form v in
let* resp, body = Client.post ~headers ~body uri in
...

For more details on constructing a Multipart_form.multipart, take a look at Multipart_form.