Maintain checkbox state on postback

Default.aspx looks like this:
<form> <!-- various ASP.NET server controls --> <input type="hidden" name="options" value="value1" \> <input type="hidden" name="options" value="value2" \> <! -- etc. --> <asp:Button runat="server" ID="MySite" OnClick="DoSomething_Click" /> <asp:Button runat="server" ID="ExternalSite" PostbackUrl="http://OtherWebsite.com/form" /> <!-- here is the problem --> <input type="checkbox" name="options" value="valueN" /> </form>  
Note that when I click ExternalSite, I am sending the form's data to another website. Therefore the name attribute *must* be "options", because that is what the  external form expects.
The problem is maintaining the state of the ch ...