Outlook Add-in: Validate form in task panel when user click on Appointment send

We are developing an Office-js add-in for Outlook that displays a form with mandatory fields in the task pane when opened. When a user opens an appointment and accesses the add-in, we need to validate these fields upon hitting “send.” If the validation fails, the send action should be restricted.

We attempted to use the OnAppointmentSend event, but even after filling the mandatory fields in the task pane, it seems that the TaskPane and LaunchEvent-OnAppointmentSend follow different flows. Please correct me if I’m mistaken.

We’ve tried various workarounds but haven’t found a solution. Is there a way to achieve this?