"Tomorrow or never!"
Translation:Yarın veya asla!
Yoksa also means otherwise.
But if you talk about its sense of 'or', yes it's only used with question sentences and it must come right after the question participle -mI.
Yarın mı yoksa asla mı? -> Tomorrow or never? (the answer looked for is only one of them - choose one of them!)
Yarın veya asla mı? -> Is it (either) tomorrow or never? (the answer looked for is whether it's one of them or not)