I just shazamed a song and the 4 months Free Apple Music popped up again. I’ve had it before so only got 1 month, new users should be able to get 4 though!
Download the Shazam app.
Shazam a song, scroll down and click on the offer.
You will need to cancel before it ends before they charge you each month and register a card with your iTunes account.
(I didn’t screenshot so images were found online)