This article shows how to synchronize a label custom field.
Let's assume you want to synchronize a label custom field named label - cust1.
Set the received label to the local label custom field and if it doesn't find one, it sets an empty array so you don't get a null pointer exception.
This method overrides all the existing labels in the custom field.
Add the received label into the local label custom field
How to Set a Default Value in a Label Custom Field?
To set a default value in a label custom field use the nodeHelper.