As @dellgreen says this image is really for testing and demo purposes. You should consider developing your own custom Golden Master image of Raspbian with SSH and other services you want enabled. Then if you use mender-convert on that image you should have what you need.
That said, specifically for the ssh service, you can use the RaspberryPi OS support for headless configuration to force ssh to be enabled. See section 3 here.
However, due to the reworking of the partitions that has happened with the Mender-enabled images, instead of creating the 0-byte file named ssh in the boot partition you need to ensure that that file exists in the /boot directory of the rootfs image in the Mender artifact. You can use mender-artifact cp to modify the artifact and add that particular file.