About tokenizer of machine translation

Hello everyone,
I am learning about machine translation tasks with transformers. To my knowledge, the transformers model predicts the next word of the target sentence based on the previous words of the source sentence.
However, in the MarianMT model (or T5), I find its tokenizer does not have a start of sentence token (<cls> or <s>). I think that token is needed to start predicting the first word in the target sentence.
Can anyone explain to me how the MarianMT model will predict the first word in the target sentence?
Thank you.