how to make enumeration type of attribute a mandatory field with pop up to select a value?

What we expect is when saving a workitem and the custom field(enumeration type) would prompt the user to select a value, similar to how file against work when it is set as mandatory(a pop up prompts the user to select the project etc). Is that possible to achieve that?
Thanks
Accepted answer

you basically set the enumeration property Unassigned Literal to the default literal e.g. "Unassigned". Once you did that the Operational behavior considers the unassigned value as not being one of the values that are considered mandatory. The default for the Priority Enumeration for the unassigned literal was None and I changed it so unassigned like below and I was prevented to save. The same should apply to custom enumerations.

Comments

Thank you very much for the answers.
I actually tested the same before posting the questions. I shut down the server and restarted it and I now see the client rejects Save when custom enumeration is set as Unassigned, I don't know why priority still not working if it is set as Unassigned(I can save). I am wondering when we add precondition, do we need to restart the server or at least logout the client to make it work?(I do see it is effective immediately when I set file against as required attribute)

Sorry, I misunderstood your info. If Unassigned literal is not set, then I can save the workitem even though the enumeration is mandatory. If I set Unassigned literal, then I don't need to restart the server or logout the client, it will consider Unassigned as non-correct value and won't allow me to save the workitem.