If your Metal Pop-Up assembly was manufactured after 1997, you’ll have to replace the drain flange and stopper. You would then attach the appropriate finish suffix to the part number. With no suffix the part will be chrome. The flange unscrews from the rest of the pop up assembly.
If your Metal Pop-Up assembly was manufactured before 1997, you may need to replace the entire pop-up assembly. The chrome drain flange is still available, but the other finishes are not. If you have another finish, you will need to replace the entire drain assembly.