Layovers under 24 hours usually don't need anything extra, provided you've got a confirmed onward ticket and stay airside the whole time. Step outside the airport, though, or your connection runs longer, and the Benin Transit eVisa becomes necessary. It's applied for the same way as other e-visas online, with a valid passport carrying at least six months' remaining validity, plus your onward ticket. Processing generally takes one to two days. Worth double-checking your specific situation against current rules before you fly, since transit exemptions can shift depending on nationality and how long your layover actually runs.