ASPxClientGanttTaskEditDialogShowingEventArgs Class

<dx:ASPxGantt ID="Gantt" >
    <ClientSideEvents TaskEditDialogShowing="onGanttTaskEditDialogShowing" ... />
</dx:ASPxGantt>

function onGanttTaskEditDialogShowing(s, e) {
    ...
    e.values["Subject"]= "Custom dialog text";
    e.readOnlyFields.push("Subject");
    e.hiddenFields.push("PercentComplete");
}