write + (past) + upon doing
kaita saini
upon doing; on the occasion of — a notable event or juncture
meaning: upon writing
念のため、出発したさいに、忘れ物がないかご確認ください。
nen no tame, shuppatsushita saini, wasuremono ga nai ka go kakunin kudasai.
As a reminder, upon departing, please check that you haven't left anything behind. formal
帰国した際に、この書類を提出してください。
kikoku shita sai ni, kono shorui o teishutsu shite kudasai.
Upon returning to your home country, please submit this document. formal