Onana kept a clean sheet as Erik ten Hag's men cruised to a 4-0 victory over Willem II on Sunday, ending a five-year trophy drought.
The 23-year-old goalkeeper started his career at the Samuel Eto'o Academy in Douala before joining Barcelona at the age of 13 in 2010.
After developing his game in Catalonia, Onana joined Ajax in 2015 and Eto'o is pleased with the latest achievement of his academy product in the Netherlands.
"Congratulations champion Andre Onana," Eto'o wrote on Instagram.
The Indomitable Lions have been paired against Guinea-Bissau, Benin and Ghana in Group F.