Warning Message to popup when DFF field is not null

Summary:

We have a DFF field on the assignment page, We want to a warning message to be popped up when the field is filled and is not null.

We are able to achieve this , but the message is very inconsistent , that it appears sometime and doesnt. Also this warning message is not present on the page untill we click on OK and acknowledge. This immediately vanishes off in less than a second.

Can you help me on how to achieve this ?

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