iPhone, iPad, Apple TV
Unfortunately Apple have recently banned all promo codes that unlock content or give things away for free. The process is now a little more complicated than it once was.
Go to Settings > FulGaz Subscription
You will then be able to add your promo code on your iPhone or ipd, or the Apple TV will email you an auto-login link to add your code
Once you've done that, restart FulGaz and you're ready to go.
If you can't see the extra things you're expecting like a trial extension or exclusive content, restart FulGaz.
Go to Settings > Promo code
enter your code