In the Directory Sync portal, click on the hamburger menu just to the right of 'DIRECTORY SYNC'
Select Templates
Locate and select the Template
Click Settings
Go to 'Mapping' tab
Double-click the attribute you wish to only be populated if/when ODM intially creates the object
Click Advanced
Go to Condition field and add the value Action = "create", as seen in the below example.
Save
Run the workflow