Just in case it helps anyone, here's what my service block looks like:

  (services
    (let ((custom-services (list (service gnome-desktop-service-type)
			         (service bluetooth-service-type)
			         (service cups-service-type)
			         (service tailscale-service-type)
				 (set-xorg-configuration
				   (xorg-configuration
				     (keyboard-layout keyboard-layout)))))
	  (nonguix-url "https://substitutes.nonguix.org/")
	  (nonguix-key "(public-key (ecc (curve Ed25519) (q #C1FD53E5D4CE971933EC50C9F307AE2171A2D3B52C804642A7A35F84F3A4EA98#)))"))
      (modify-services
	(append custom-services 
		%desktop-services)
	(guix-service-type config =>
			   (guix-configuration
			     (inherit config)
			     (substitute-urls
			       (append (list nonguix-url) 
				       %default-substitute-urls))
			     (authorized-keys
			       (append (list (plain-file "nonguix.pub"
							 nonguix-key))
				       %default-authorized-guix-keys)))))))