In your HubSpot account, navigate to Marketing > Lead Capture > CTAs.
In the upper right, click Create CTA, or hover over an existing CTA and click the Actions dropdown menu and selectEdit.
In the right panel, rich text editor, click the source code icon code.
Please note: <i class=...> elements cannot be used with the source code of HubSpot's rich text editors. When you are implementing Font Awesome icons, it's recommended to use <span> tags as noted in the example below.