Outlook 365: Appointment Form change

Hello,

I'm working on a project to extend the appointment form of Outlook 365.

My first idea was to build an Add-In.

But with this I have the issue that I can't pin the side pane on all Outlook 365 on Windows and Mac.

Is there I way to pin the pane that I'm not aware of?

Or is there another way to add fields to the appointment form and use them in the add-in?

Thanks a lot for answers in advance.