The affix ṭhak comes after a word denoting a condiment, being in the Instrumental -3rd case in construction when the sense is 'sprinkled therewith'.,
Thus दध्ना उपसिक्तं = दाधिकम् 'sprinkled with curd', सौपिकम्, खारिकम् ॥ Why do we say 'denoting a condiment'? Observe उदकेनोपसिक्तं ओदनः ॥,