auto select multip combobox items on HTML Page

I am trying to fill out a form on a webpage. It has a dropdown/combobox with over 50 possible selections. I am able to write the code to select 1 item using:

.SetAttribute("selectedindex", "2")

However, I need to select 2 items from the list. Can someone help me figure this out. I have just about exhausted all my resources trying to solve it.

I am using vb express vb

Thanks