We have an add in that works with user properties set as Outlook.OlUserPropertyType.olInteger I have been asked to integrate that with MSGraph.
According to this post if user properties are created in a VSTO add in this can be accessed through MS Graph but how can I go the other way? I need to use Outlook extended properties but I am not sure how
So far I am not sure how to add extended properties and how to pick them up in a VSTO add in
You need to use the GUID for dealing with user properties. See How do I retrieve Extended Properties from a Calendar Event using MS Graph? for more information. For example:
https://graph.microsoft.com/v1.0/me/events('AAMkADU4MzkxN2RmLTdiZDAtNDIwYS04NjQzLTUzNzMyMjM0Y2VkNQBGAAAAAABGjw0ByCaySL6aUxJmew3qBwDwiT27qO5xT6RMWiWBhwRzAAAADIqqAAAYpSJ8f1CATrNsRrtUbquzAAEOAONsAAA=')?$expand=singleValueExtendedProperties($filter=id%20eq%20'String%20{00020329-0000-0000-C000-000000000046}%20Name%20MyCustomData')
User properties are twofold: their definition is stored in a special blob (you can see its definition in MFCMAPI or OutlookSpy (I am its author) - click IMessage button) and their value is stored as a regular named MAPI property (see the OutlookSpy screenshot in Eugene's answer). The blob format is documented, but parsing or (even worse) creating it is a major pain. If you don't need the property to be present in the MailItem.UserProperties collection, you can read or set its value using MailItem.PropertyAccessor.GetProperty/SetProperty by the property DSL name (also shown by OutlookSpy). To read or set the value using Graph, you can use the link in your question.