iCloud Mail

Plain-text signature on iCloud.com

iCloud Mail in a browser supports a plain-text signature. The designed layout, images, and colors from Sigxel will not be preserved there.

Open iCloud.com → Mail → Settings → Composing, enable Add a signature, and enter your name and contact details as text.

To use a formatted Sigxel signature with your iCloud address, set the account up in Apple Mail on Mac, then install the signature in that app.

Source: Apple's iCloud Mail signature guide.