Spark | Signature Designer

Steps

In Email Signature Designer : Install > Copy to clipboard ;
Open Spark app ;
Open Preferences… panel ;
Select the Signatures tab ;
Add Signature ;
Paste the signature in the text area with the keyboard shortcut cmd+V ;
In the above section, you can set Default for… to automatically pre-fill the signature when writing a new message.

You can now compose a message with your new signature.