How to configure to display the message if one of the DFFs is not having value

Summary:

We have enabled DFF on the work order operation page to capture the quality Nonconformance Number.

DFF1 : Rework = Yes

DFF2 NC Number = 25

In Case , if user forgets to enter the Value for DFF2 (NC Number) Business would like to have system display a message " Please Enter NC Number", when they click on OK(B)

form validation should check Rework = yes and NC Number is Null, then display message

we cannot make DFF mandatory, because it will impact all the operations, hence we are looking on UI validation

Appreciate if any of the members have done this customization to display a message, if so Please share the steps