No. Réunion doesn't ask visitors to fill in an online arrival form (also called an entry form, immigration card or landing card) before they travel. French overseas department; French entry rules apply. No online arrival form found.
Do I still need a visa for Réunion?
An arrival form is not a visa. Whether you need a visa or an electronic travel authorization depends on your nationality, so check Réunion's official immigration website or your own government's travel advice.
Other countries in East, Central and Southern Africa