Is it permissible to send a follow request to a young man to get his attention, knowing there is one-sided love, while being careful not to fall into a forbidden relationship?

You must be careful not to send a follow request to a young man to get his attention, as it could be an opening for Satan and a pretext for falling into the forbidden. Allah has forbidden following the footsteps of Satan. If the young man is religious and of good character, and might be amenable to marriage, then you can inform his sister or one of his female relatives of your desire to marry him, for it is permissible for a woman, according to Sharia (definition), to seek suitors. If marriage becomes easy, then that is good; otherwise, you must turn away from thinking about him and ask Allah for someone better, and the heart's attachment to him can be treated.