What is the intention for the supererogatory prayer? May Allah reward you with goodness.

Scholars have divided supererogatory prayers (Nawafil) into two categories: designated and absolute. Designated prayers include Witr, Salatul-Istisqa (prayer for rain), and the regular Sunnah prayers (Sunan al-Rawatib). In these, the specific prayer must be designated. As for absolute supererogatory prayers, it suffices to merely intend to pray, and the place of intention is the heart.