Get custom property set in Outlook Add-In via Microsoft Graph

Question

Let\'s say I have in my Outlook Add-In (using Office.js) following code running on a compose form of an appointment:

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Answer 1

TL;DR; Yes it is possible to read custom properties set by Office.js api loadCustomPropertiesAsync using REST APIs You need to create a REST call query that looks like

string addinManifestId = "";//lower cases
string prop = @"String {00020329-0000-0000-C000-000000000046} Name"+ string.Format(" cecp-{0}", addinManifestId );
var url = $"/messages/?$expand=SingleValueExtendedProperties($filter=PropertyId eq '{propertyname}')";

More info

The documentation is available here https://msdn.microsoft.com/en-us/library/hh968549(v=exchg.80).aspx

I had a similar problem when I needed to handle "sent mail" for my add-in Keluro. I implemented something really close to your approach with a web hook on REST API. To my knowledge it is the only way to handle sent items. Consider upvoting this https://officespdev.uservoice.com/forums/224641-feature-requests-and-feedback/suggestions/10964871-add-itemsend-event-so-add-in-can-cancel-email