@Charlie Horton This is possible under the Microosft.graph namespace only cannot be used for any custom app. If you have a requirement for getting any custom property for users, groups, device organization or application you can use extension property resource type to achieve that.
For example, if an organization has a line of business (LOB) application that requires a Skype ID for each user in the directory, Microsoft Graph can be used to register a new property named skypeId on the directory’s User object, and then write a value to the new property for a specific user.
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