This guide will help you to get the HTML source code, which you can further paste to Thunderbird, Roundcube, Hubspot, and other environments that support HTML.
Here are three necessary steps:
From the Dashboard, hover over your signature and click Install button.
Or by clicking the Save signature button if you are in the generator.
Select the Source code in the list of email clients and click Copy source code button.
Now you can paste your copied source code to the signature box of your email client, which requires pasting the HTML source code.