If you want to confirm before process with ASPxButton in Devexpress, then you can try the code below:
<dx:ASPxCallback ID="ASPxCallback" runat="server" ClientInstanceName="ASPxCallback">
<ClientSideEvents CallbackComplete="function(s, e) { LoadingPanel.Hide(); }" />
</dx:ASPxCallback>
<dx:ASPxLoadingPanel ID="LoadingPanel" runat="server" ClientInstanceName="LoadingPanel" Modal="True" >
</dx:ASPxLoadingPanel>
<dx:ASPxButton ID="btnDelete" runat="server" Text="DELETE ITEM" OnClick="btnDelete_Click">
<ClientSideEvents Click="function(s, e)
{
e.processOnServer = confirm('Are you sure you want to delete this item?');
ASPxCallback.PerformCallback();
LoadingPanel.Show();
}"
/>
</dx:ASPxButton>