Send string with QueryString in Repeater Control in ASP.net

In the method for the button click:

foreach(RepeaterItem item in repeaterControlID.Items)
{
    var quantity = item.FindControl("quantityControlID");
    var sku = item.FindControl("skuControlID");
}

Or something like that. You could alternatively use the RepeaterItem's Controls property to look through them.

E: my interpretation of your question was: "On the postback caused by a button click, I want to loop through the items in the repeater and pass those values into a method." This will not be applicable to populating the repeater or doing something in client-side javascript.

And of course, MSDN is a great resource for learning how to use various classes, like the Repeater

Though your code should work fine, even if the source string has spaces etc. it should return something when you access query string, please try this also:

protected void FReplybtn_Click(object sender, EventArgs e)
{
    String s = Page.ResolveClientUrl("~/ADMIN/Reply.aspx");
    s += "?Subject=" + Server.UrlEncode(FSubjectlbl.Text.ToString());
    Response.Redirect(s);
}

EDIT:-

void Page_Load(object sender, EventArgs e)
{
    if(Request.QueryString.HasKeys())
    {
        if(!string.IsNullOrEmpty(Request.QueryString["Subject"]))
        {
            RSubjectlbl.Text += Server.UrlDecode(Request.QueryString["Subject"]);
        }
    }
}

PS:- Server.UrlEncode is also sugested in comment to this question.