Retrieve Values from a ListBox

Hello, I have this problem,
I have populated a multiselect ListBox, but now I need to retrieve
selected values, and use them as a parameters on the url e.g
http://www.aaa.com/Dynamicpage.aspx?states=(value or values) - which
will be selected from this ListBox, how do I separate multiple values
when they get selected?
Thank you

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