How to make a custom field value unique

But I think the Work item ID is not enough for me , I want use another field to indentify a workitem other than the ID, so I can use other type or format , like add some prefix or suffix ,such as XXX-01,XXX-02, and I can customize how it looks like

hi,

Then what you can do is that Re-Use the Work item ID and Prefix some text with that value itself. Let say WI id is 1001, then you can take the ID and Prefix some text as XXX-1001.

Same can be done with Java Script - Calculated Values.