Open your HTML email signature file in your web browser.
Press Ctrl + A to select the entire email signature, then press Ctrl + C to copy it.
IMPORTANT: Do not use your mouse to select the area, only use Ctrl + A as you may miss out on grabbing some of the code.
Login to Fastmail in your web browser and select the Settings icon at the top right, then select View All Settings.
From your left menu under Set up, select Signatures & Compose.
Click Edit on the default email signature that Fastmail has created for you.
Click inside the signature field and press Ctrl + V (Cmd + V for Mac) to paste your copied email signature. Hit Save.
Close out of your settings and compose a new email. Your email signature will now automatically insert when sending emails from Fastmail.