webbrowser.SetAttribute for multiple values?

[xzibited2]

Guys - Having a hard time figuring this one out, and have so far been unsuccessful. I'm manipulating a web form that has a listbox where I need to select multiple values. Here is the line that successfully selects one of the values:

WebBrowser1.Document.GetElementById("Office").SetAttribute("Value", 1)

Any ideas how to select multiple values?