Here's the code that I used in Thunderbird. I make it small and light in color.

<div style="font-size: 10px; color: #666;">This message is signed using <a href="https://en.wikipedia.org/wiki/Pretty_Good_Privacy">PGP</a>. Public key: <a href="https://pgp.mit.edu/pks/lookup?op=get&search=0xBE5D1E31ABCD1... ABCD1234</a> </div>

I see the link in the code is not closed properly, so you need to fix this.