添える
そえる · soeru · JLPT N1 · verb-ichidan
to add to
添 — water radical 氵 + 忝 (phonetic)
添 is a phono-semantic compound. The water radical 氵 hints at the meaning 'to add water, to pour in more,' while 忝 supplies the sound. From the literal sense of topping up liquid, it broadened to mean 'to add' or 'to append' anything.