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