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:
Go to the signature preview page by clicking Preview and install button; if you are in the Dashboard,
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 that requires pasting the HTML source code.