If you have the data needed to create the link when generating the initial HTML, you can do something like this in the Page_Load event:
protected void Page_Load(object sender, EventArgs e)
{
Button1.OnClientClick="javascript:window.open('MyPage.aspx?Param=" + Param1.ToString() + "');"; }
}
If you're waiting for the PostBack to get the required data to build the link, you can send javascript down to the browser via the ScriptManager:
protected void Button1_Click(object sender, EventArgs e)
{
//process whatever you need to to get Param1
ScriptManager.RegisterStartupScript(Page, typeof(Page), "OpenWindow", "window.open('MyPage.aspx?Param=" + Param1.ToString() + "');",true);
}