Go to settings > Beaver Builder
Select the Beaver Extender tab
Paste your API key in the setting dialog
The buttons module is very similar to, and inspired by the buttons module in Beaver Builder Pro. However, with a few opinionated differences. We weren't happy with the way way the built in buttons behaved in certain situations, and we were less than pleased with the difficulty styling the default buttons for our themes.
Many of the same options are available in our buttons module as there is in the core version:
Icons / Icon Options