Hello,

Normally, you could not get it from server side because there is no method or property for this scenario. 
It only can be done via javascritp. However, you can pass the data from client(javascript) to server.

E.g

function WebGrid1_OnAfterResponseProcess(controlId, actionName, lastRequestObject, xmlResponseObject)
        {
            if (actionName != "Custom")
            {
                var WebGrid1 = ISGetObject(controlId);
                var pageIndex = WebGrid1.CurrentPageIndex;
                WebGrid1.AddInput("pageIndex", pageIndex.toString()); 
                WebGrid1.SendCustomRequest();
            }
            return true;
        }

 protected void WebGrid1_InitializePostBack(object sender, ISNet.WebUI.WebGrid.PostbackEventArgs e)   {
     if (e.Action == ISNet.WebUI.WebGrid.PostBackAction.Custom)
     {
          pageIndex = Request["pageIndex"] as string;
     }
   }

Regards,
Handy 

How do I set the grid to load that specific page in the code behind instead of loading the first page?

Hello,

This cannot be done in server side. You only can do it in client side with OnAfterInitialize event and run below code.

var grid=ISGetObject("WebGrid1");
grid.GotoPage(2);

Regards,
Handy