Need some help in BPM

Hi everyone. Currently i'm trying to validate a new field that i added
in the Journal Entry called Journal Reference Number. The new field
can only be added once. Can someone guide me through on how to do this
validation? Do i use BPM or the customization script editor to do it?
Thank in advance and im new in Vantage. Hope someone can help me out.

Best Regards
Adrian Chang