Hi @Afd-mark,
It’s not possible to define how exactly a form field should update the value of a property. If this is a recurring issue, I’d recommend creating an additional field, e.g. ‘First name (for forms)’, which can be copied via contact-based workflow into the actual first name field if the actual first name field is empty.
This would of course still have a manual element but it would ensure that a known first name is not overwritten.
Let me know if you need help with the workflow, happy to draft it here.
Best regards!