Set up your email account(s) using the Spark tutorial:
To set up your signature, simply go to the Preferences menu:

Select the ‘Signatures’ tab:

Tick the “Enable signatures” box, then click the + button at the bottom left of the window:

In your user interface, under the “Email signatures” section, select and copy the “HTML code” displayed below the signature:

In Spark, click the “HTML” button and paste your code:

That’s it – your signature is now set up and will be automatically updated whenever you make changes in future.

When you exit “HTML” mode, you can also add text to your signature.
There you go, your signature is set up and will be automatically updated whenever you make future changes.
All that remains is to test your signature by sending yourself an email BUT PLEASE NOTE: the link in your signature is NEVER active whilst you are editing the email; it only becomes active once you’ve received it.
Email signature – Spark on macOS