I've created a form to select specific meeting hours. I call my add-ins from AppointmentOrganizerCommandSurface extension point. My form is in a displayDialog. When the user confirm his choice (time, date, details, ...), the dialog close and all his information are copied in Outlook Appointment composer form.
Is there a function I can call to create appointment when the dialog box close and the add-ins call event.completed()? To avoid the need to confirm the appointment again.
(moving previous comment to an answer)
No, there's no JS API to 'send' the item from the addin. Generally, we want to let the user send from the native UI.
This is not what you asked, but would the on-send learn.microsoft.com/en-us/office/dev/add-ins/outlook/… be applicable in your scenario in that you trigger your dialog when user sends? REST/EWS is a possibility, but on desktop Outlook, what you set and save in the composer will take time to sync to the server.