About Transformer task prefix

Hi all,

I am a new user, please forgive me if my question is too silly, but I tried many searches and still cannot find a satisfactory solution so here I am.

I want to fine-tune the T5 model with my sequence to sequence task that take one sentence as input and generate another sentence correspondingly. I read many examples from huggingface community notebooks and other sources, and i found there is a prefix added to the input or target.
prefix at input article.
prefix at output notebook.

Can I build whatever prefix by myself ? e.g. ‘paraphrase_sentence’, or it is a something already fixed by authors ?
I also want to know where should I add the prefix for my case (input or output) ? and what prefix should I add for my case?

my data is like:
sentence_a1, sentence_b1,
sentence_a2, sentence_b2,

and I would to train a model that can generate a sentence to a given sentence.
something similar to paraphrase.