set Multiple select

set Multiple select

Hello,

I'm new here and have a problem.
When I've a select box I can set the selection like
  1. $('#selectbox').val(2);

Now I've a multiple select. What I want to do is to select more then one option like this.
  1. $('#selectbox').val(2, 4);
This example select only the first option but not the second.

Thanks for your Help