To create a “share” button in Builder:
Drag and drop a button block onto your page.
Select the button and click Edit or navigate to the Options tab.
In the Link field, enter in:
mailto:?subject=Check out this awesome page!&body=Hi! Check out this awesome page: https://builder.io/
Now when someone clicks on this button, it will open their default mail client on their computer with the subject and body filled in.