How to propogate the refnum information from Declaration page to Declaration line page?

Summary:

I have created a refnum at Declaration level and same refnum field at Declaration line level. At header level on Declaration page there is write access so we can change that refnum value.
But once the refnum value is changed, it is not letting me propogate that value from Declaration to Declareation line. I have tried using agent with Update/Insert action.

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