Confirmation Box

I want a confirmation yes/no  box to present itself within the loop. It should appear at the end of this code snippet. Can you help?
       protected void btnMarkAsReceived_Click(object sender, EventArgs e)         {             foreach (GridViewRow gr in uxReconGridView.Rows)             {                 CheckBox chk = (CheckBox)gr.FindControl("uxSelectBox");                 if (chk.Checked)                 {