Hi Queyen,
Thank you for reaching out to the QnA community.
If you would like to sync any attribute which does not supports direct sync, you would need to use Directory Extension Attribute Sync: https://docs.microsoft.com/en-us/azure/active-directory/hybrid/reference-connect-sync-attributes-synchronized.
When you use "Directory Extension Attributes" in AD Connect Server these new attributes are synced to AAD. However, these new Attributes would not be visible on GUI.
You would need to use Graph to query and view these attributes on the users in AAD.
The document below has more details about viewing directory extension attributes.
I hope this helps to resolve your query. Please "Accept the answer" if the information helped you. This will help us and others in the community as well.