All you need to do is follow this link.
Or, if you’d prefer to sign up for the iHeartRadio All Access trial on your Android or iOS device, just launch the app, open the menu on the homescreen and tap the Upgrade Now button!
Note: One free trial is permitted per iHeartRadio account, credit card, Apple ID, and Google Play Store account.
For more information on how to sign up for a free trial on your device click here.
To read about subscription terms and conditions click here.