Open your HTML email signature file in your iPhone's web app.
Tap and hold on the name in your email signature, then tap the Select All option, then Copy.
Open the Canary Mail app and tap the icon at the top left of the app. Select Settings from the sidebar.
Scroll down and select Signatures. Select your email account from the Signatures list.
Tap inside the signature field and delete the text that says "Sent from Canary". Tap and hold inside the field again, and select Paste. Hit Save at the top right.
Close out of your settings and compose a new email. Your email signature will now be automatically inserted when sending emails from the Canary Mail iOS App.