Obtained: Speak with Elder Kai in the Land of the Kais during the post game as Goku. It unlocked for me after completing the Reunion with Suno and Good Ol’ Goku! sub stories.
Objective: Fulfill Goku’s promise
Travel to the Land of the Kais to find Elder Kai and speak with him to trigger a cutscene. When you regain control of Goku, head out to collect all seven Dragon Balls then return to Bulma when you have them all to trigger another cutscene, completing the quest.