How to sync extension attribute from on-prem AD to AAD - and how to use it

ADC syncs a default set of attributes from AD to AAD out of box. To sync extension attributes (the term 'extension attributes' here doesn't refer to "extensionattribute1-15", but rather any attribute that is not included by default by ADC for sync with Azure):

1. Open ADC | Configuration | Customize Sync Options
2. In "optional features", check "Directory extension attribute sync"
3. In attribute select page, check and select what you want to include to be sync'ed

Within Azure AD portal, how to retrieve extension attribute

1. There should be a new app called "Tenant Schema Extension App" , this is where you can find App ID so you can retrieve extension attributes
2. E.g. Creating a dynamic group based on extension attribute
1. when create group rule, select "Get custom extension properties" option
2. In new window, paste in the App ID you copied from step 1, then click on refresh button
3. back to attribute dropdown list, at buttom the new attribute should be available for you to chose. The attribute name is in format similar to "extension_appID_GUID_onPremAttName"

See Also: general concept of extend attribute in AAD