What’s the simplest way to call Http POST url using Delphi?

Using Indy. Put your parameters in a StringList (name=value) and simply call Post with the URL and StringList.

function PostExample: string;
var
  lHTTP: TIdHTTP;
  lParamList: TStringList;
begin
  lParamList := TStringList.Create;
  lParamList.Add('id=1');

  lHTTP := TIdHTTP.Create;
  try
    Result := lHTTP.Post('http://blahblahblah...', lParamList);
  finally
    lHTTP.Free;
    lParamList.Free;
  end;
end;