program JSONPostExample;

{$APPTYPE CONSOLE}

uses
  IdHTTP, SysUtils, Classes, IdGlobal;

var
  HTTP: TIdHTTP;
  RequestBody: TStringStream;
begin
  HTTP := TIdHTTP.Create;
  try
    try
      RequestBody := TStringStream.Create('{"日本語":42}', TEncoding.UTF8);
      try
        WriteLn(HTTP.Post('https://httpbin.org/post', RequestBody));
        WriteLn(HTTP.ResponseText);
      finally
        RequestBody.Free;
      end;
    except
      on E: EIdHTTPProtocolException do
      begin
        WriteLn(E.Message + #13#10#13#10 + E.ErrorMessage);
      end;
      on E: Exception do
      begin
        WriteLn(E.Message);
      end;
    end;
  finally
    HTTP.Free;
  end;
  ReadLn;
  ReportMemoryLeaksOnShutdown := True;
end.

{
  "args": {},
  "data": "{\"\u65e5\u672c\u8a9e\":42}",
  "files": {},
  "form": {},
  "headers": {
    "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",

    "Accept-Encoding": "identity",
    "Content-Length": "16",
    "Host": "httpbin.org",
    "User-Agent": "Mozilla/3.0 (compatible; Indy Library)"
  },
  "json": {
    "\u65e5\u672c\u8a9e": 42
  },
  "origin": "80.137.6.116",
  "url": "https://httpbin.org/post"
}

HTTP/1.1 200 OK