Both prepared horseradish and horseradish root can work. Prepared horseradish is easier to use as it's "prepared" for you, finely chopped or grated and pickled in vinegar. If it's juicy, drain off the vinegar so as not to add too much liquid to your sauce. If you prefer fresh horseradish root, be aware it's much hotter and spicer than prepared. Peel the root and very finely grate. Start with half as much (or less) and build up to your preferred heat level.