Instructions cover Mail in macOS Catalina through OS X Lion.
Click theSignaturestab at the top of the MailPreferencesscreen.
SelectAll Signaturesor choose the account with a signature you want to edit in the left column of the screen.
Select thesignaturefrom the middle column.
In the right panel, highlight the text to which you want to add a link in your signature.
SelectEdit>Add Linkfrom the menu bar, or use the keyboard shortcutCommand+K.
Enter the complete internet address for the link,including http://in the field provided, and clickOK.
Drag the signature fromAll Signaturesto the account you plan to use it with.
