When you unlock the Neymar Jr skin by completing 5 challenges given by the soccer player NPCs located at Pleasant Park, Dirty Docks or Holly Hedges. The challenges are very simple and include travelling distance by foot and using the soccer ball toy to score goals at the near by football goal.

When you have the Neymar Jr skin unlocked equip it and in your next game go back to one of the three locations mentioned above where you can find the soccer player NPCs. Use the soccer ball toy again to score a goal and you will complete this quest.

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