You could add it to the
msg for example, extending the example from the official docs you could use:
msg := byte("From: the name <firstname.lastname@example.org> " + "To: email@example.com " + "Subject: discount Gophers! " + " " + "This is the email body. ")
From: the name <firstname.lastname@example.org>
The msg parameter should be an RFC 822-style email with headers first, a blank line, and then the message body. The lines of msg should be CRLF terminated. The msg headers should usually include fields such as "From", "To", "Subject", and "Cc".