How does a person complete their prayer if they forget to recite the qunut supplication in the witr prayer?

In its lexical sense, qunut means to worship Allah with sincerity, to prolong prayer and supplication, to remain silent, to pray, and to stand in worshipful intent. As a religious term, it refers to making supplication while standing in prayer, either before or after the bowing (ruku‘). Reciting the qunut supplication in the witr prayer is obligatory (wajib). Therefore, if the qunut supplication is omitted or delayed, it is necessary to perform the prostration of forgetfulness (sajda sahw) (Haddâd, al-Jawhara, 1/77). If a person performing the witr prayer forgets or mistakenly goes into ruku‘ without reciting the qunut supplication, they should continue their prayer and perform the prostration of forgetfulness at the end. However, if this person recites the qunut supplication after rising from ruku‘, they should proceed to prostration without repeating the ruku‘, and again perform the prostration of forgetfulness at the end of the prayer (Kāsānī, Badā’i‘, 1/274; al-Fatāwā al-Hindiyya, 1/111).

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu