The Font Awesome icon shortcode has 3 options:

Icon: Choose the icon
Example: [qt_fa icon="fas fa-cog"]

Href: Add a URL to the icon
Example: [qt_fa href="https://yourdomain.com"]

Target: Link the button to a new window, or not. To link in the same use _self or new _blank window
Example: [qt_fa target="_self"]

Size: Change the font size of the icon
Example: [qt_fa font_size="16"]

On the Font Awesome website you can see all available font classes you can use.