Conditional format a cell if value is found in list on another sheet

Good day, Gurus.

Not sure if this can be done. Column T in my workbook requires an entry, on any number of rows, for the country of origin. The entry in that column must exactly match the abbreviation for that country, which I have stored in a list on a separate hidden worksheet.

Do I use conditional formatting or data validation to color this cell red if a user enters a value not found in my list, and can you please offer an example of how to best accomplish it?

Thanks in advance for any help you can offer.