What I would do is download the files from FontAwesome, then upload the 2 files in the CSS folder from that Zip file into your file manager. Once it is in HubSpot copy the URL. Now that you have the URL go to Content > Content Settings, Then add this to the Site Header HTML section. <link rel=“stylesheet” href=“path/to/font-awesome/css/font-awesome.min.css”> and replace what is in the quotes with what you copied previously for the font-awesome.min.css file in the file manager.
Now that this CSS is firing on your site you should be able to use any one of those icons by using the <i class=“fa fa-id-card-o” aria-hidden=“true”></i> as an example.
Alternatively, you can check out this HubSpot ariticle. https://knowledge.hubspot.com/articles/kcs_article/cos-general/how-to-install-font-awesome keep in mind it does not have an up-to-date list of the current icons.