(with-eval-after-load 'org
  (defun my-org-notmuch-show-store-link ()
    "Store a link to a Notmuch message."
    (interactive)
    (require 'notmuch)
    (when (eq major-mode 'notmuch-show-mode)
      (org-store-link-props
       :type "notmuch"
       :link (concat "notmuch-show:" (notmuch-show-get-message-id t))
       :description (notmuch-show-get-subject)))))