This article shows how to synchronize a label custom field.

Source side

Outgoing sync

//Assume you want to synchronize the label custom field 'label - cust1'
replica.customFields."label - cust1" = issue.customFields."label - cust1"

Destination side  

Incoming sync 

Add the script below the Create/Change Processor.

Please note!

This approach will overwrite all the existing labels in the custom field.

issue.customFields."label - cust1".value = replica.customFields."label - cust1"?.value 

See Also