o.AddPair('key', 'value');

    try
      RESTClient.BaseURL := edtAddress.Text;
      RESTRequest.Params.AddHeader('username', 'devtest90@mail.ru');
      RESTRequest.Params.AddHeader('password', '123456');

      RESTRequest.AddBody(o);
      RESTRequest.Execute;
      if Pos('200 OK', RESTResponse.StatusText) = 0 then begin
        MessageDlg('Ошибка при отправке на сервер. Детали: ' + RESTResponse.StatusText + ': ' + RESTResponse.Content, mtError, [mbOk], 0);
        Exit;
      end
      else begin
        //Showmessage(RESTResponse.StatusText);
        mResponse.Text := RESTResponse.Content;
      end;
    except
      on E : Exception do begin
        Showmessage(e.Message);
      end;
    end;

{"args":{},"data":{"key":"value"},"files":{},"form":{},
"headers":{"x-forwarded-proto":"http","x-forwarded-port":"80","host":"postman-echo.com","x-amzn-trace-id":"Root=1-5fa04515-425528084b7b063b28e1e1de",
"content-length":"15",
"content-type":"application/json",
"USERNAME":"devtest90%40mail.ru",
"password":"123456",
"accept":"application/json",
"accept-charset":"UTF-8",
"accept-encoding":"identity",
"user-agent":"Embarcadero RESTClient/1.0"},
"json":{"key":"value"},"url":"http://postman-echo.com/post"}

RESTRequest.Params.ParameterByName('username').Options := [TRESTRequestParameterOption.poDoNotEncode];