Adding damage to Wild Shape forms?

What's the syntax for a homebrew magic item to add +1 Fire Damage to attacks while in Wild Shape? I've tried Bonus - Natural Attacks, Bonus - Unarmed Strikes, Damage - Fire. None of them actually change the stats on the Wild Shape cards. 

Thanks!

I don't think you can actually do it with an item, or with anything else. The Creatures is an environment apart from the rest of the sheet.

It might work just as a Custom Attack in the Attack section.

That is disappointing, to be honest. Oh well, I guess our Druid will continue to hand-add the attack and damage to his attacks. Cheers!