On discord, I have automated giveaways that run daily. Sometimes it's gems, other times it's shards. Shard is the name I use for the currency in endless mode. It's free, and there's no weird strings attached, I just think it's cool to get free stuff in a game.

To join, you just need to press the enter button at the bottom. If you aren't registered, you'll need to register. A button to register will appear. When you press it, you'll need to add your playfab id. You can press the playfab button in settings -> account to copy it.