if (e.Row.RowType == DataControlRowType.Header)
        {
            GridView HeaderGrid = (GridView)sender;
            GridViewRow HeaderGridRow =
            new GridViewRow(0, 0, DataControlRowType.Header,
            DataControlRowState.Insert);
            TableCell HeaderCell = new TableCell();
            HeaderCell.Text = "Name of Person";
            HeaderCell.Font.Bold = true;
            HeaderCell.ColumnSpan = 1;
            HeaderGridRow.Cells.Add(HeaderCell);
            HeaderCell = new TableCell();

            Button btnHeader = new Button();
            btnHeader.UseSubmitBehavior = false;
            btnHeader.Click += new EventHandler(this.btnsubmitentry_Click);
            btnHeader.Text = "Add New Task";
            btnHeader.CommandName = "AddNewTask";
            //  OnClick="btnsubmitentry_Click"
            btnHeader.CssClass = "btn btn-lg btn-default";
            btnHeader.OnClientClick = "AddNewTask();";
            HeaderCell.Controls.Add(btnHeader);
            HeaderCell.ColumnSpan = 2;
            HeaderGridRow.Cells.Add(HeaderCell);

            grdViewSummary.Controls[0].Controls.AddAt(0, HeaderGridRow);
        }

<input type="button" name="grdViewSummary$ctl01$ctl00" value="Add New Task" onclick="AddNewTask();WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("grdViewSummary$ctl01$ctl00", "", true, "", "", false, true))" class="btn btn-lg btn-default"   />

<script type="text/javascript">
    function AddNewTask() {
        alert();
        __doPostBack('btnsubmitentry_Click','')
    }
</script>