The ‘OpenPGP’ header can be used to provide information about the sender’s OpenPGP key. This is a formalization and modernization of the non-standard ‘X-PGP-Key’ (etc.) headers that have been in use for a long time. For more details, see https://tools.ietf.org/html/draft-josefsson-openpgp-mailnews-header.
To use this in Message, say:
(add-hook 'message-header-setup-hook 'message-add-openpgp-header)
then customize the message-openpgp-header variable according to
your PGP setup. The variable is a list of the key ID, the key URL or
ASCII armored key, and the protection preference, one of
‘"unprotected"’, ‘"sign"’, ‘"encrypt"’ or
‘"signencrypt"’.