In this video

Pull out the detergent dispenser drawer from the washing machine. Using a damp microfiber cloth saturated with the vinegar/water/Dawn solution, thoroughly clean all compartments of the dispenser, including the main detergent section, fabric softener section, and any pre-wash compartments. Use the old toothbrush to scrub away any hardened detergent residue or mold in tight corners. Once clean, wipe dry and reinsert the drawer.