How to make the client side program get an HTML element from the server and insert it in the page?

A very convenient way to do that is to use server functions :

let get_mydiv () = div [ ... ]

let%client get_mydiv_rpc =
  ~%(server_function [%derive.json: unit] get_mydiv)
...

[%client
  ...
  let%lwt mydiv = get_mydiv_rpc () in
  Dom.appendChild parent (To_dom.of_element mydiv)
  ...
]

Server functions take exactly one parameter.

Server functions are just syntactic sugar for non-attached coservices returning OCaml values.

The type of the function parameter must have been declared with the JSON module of ppx_deriving. This enables server-side data validation.

example:

type t = int * string [@@deriving json]