Attach js from server side

On suggestion is to use GridButtonColumn instead of GridTemplateColumn and set its DataTextField to corresponding field name. And you can set its CommandName and ConfirmText property like below.

ASPX:

<telerik:GridButtonColumn CommandName="Command" DataTextField="DelFlag" ButtonType="PushButton"
UniqueName="btnStatus" ConfirmText="Are you sure want to update status !!?">
</telerik:GridButtonColumn>

Another option is attaching 'onclick' client event from code behind and invoking the ajaxRequest from client event handler. You can pass the Item index as parameter of ajaxRequest. In AjaxManager_AjaxRequest server event, call the FireCommandEvent with appropriate
CommandName. Sample code is given below.

ASPX:

<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server" OnAjaxRequest="RadAjaxManager1_AjaxRequest">
<AjaxSettings>
<telerik:AjaxSetting AjaxControlID="RadGrid1">
<UpdatedControls>
<telerik:AjaxUpdatedControl ControlID="RadGrid1" />
</UpdatedControls>
</telerik:AjaxSetting>
</AjaxSettings>
</telerik:RadAjaxManager>

<telerik:RadGrid ID="RadGrid1" runat="server" OnItemCommand="RadGrid1_ItemCommand" OnItemCreated="RadGrid1_ItemCreated" >
<MasterTableView>
<Columns>
<telerik:GridTemplateColumn DataField="DelFlag" HeaderText="Status" SortExpression="DelFlag"
HeaderStyle-HorizontalAlign="Center" UniqueName="DelFlag" Visible="true" AllowFiltering="false">
<ItemTemplate>
<asp:Button ID="btnStatus" Font-Size="11px" Width="65px" runat="server" Text='<%#Eval("DelFlag")%>'
CommandName="Command" />
</ItemTemplate>
<ItemStyle VerticalAlign="Middle" HorizontalAlign="Center" />
</telerik:GridTemplateColumn>
</Columns>
</MasterTableView>
</telerik:RadGrid

Attaching 'onclick' client event from code behind by passing item index to event handler.

C#:

protected void RadGrid1_ItemCreated(object sender, GridItemEventArgs e)
{
if (e.Item is GridDataItem)
{
GridDataItem item = (GridDataItem)e.Item;
Button btn = (Button)item.FindControl("btnStatus");
btn.Attributes.Add("onclick", "return clickbtn('"+item.ItemIndex+"');");
}
}

JavaScript:

<script type="text/javascript">
function clickbtn(index) {
confirm('Are you sure want to update status !!?');
$find("<%=RadAjaxManager1.ClientID%>").ajaxRequest(index);
}
</script>

C#:

protected void RadAjaxManager1_AjaxRequest(object sender, AjaxRequestEventArgs e)
{
GridDataItem item = (GridDataItem)RadGrid1.Items[e.Argument];
item.FireCommandEvent("Command", String.Empty);
}