Panther probably give you a detailed answer
Only the relevant part from the rulebook (Europe 1940.2 page 21) ;-)
Transporting Multinational Forces: Transports
belonging to a friendly power can load and offload
your land units, as long as both powers are at war.
This is a three-step process:You load your land units aboard the friendly
transport on your turn. The transport’s owner moves it (or not) on
that owner’s turn. You offload your land units on your next turn.