Add the following udev rule to config.json:
"os": {
"udevRules": {
"66": "ACTION==\"add\", SUBSYSTEMS==\"usb\", SUBSYSTEM==\"block\", ENV{ID_FS_USAGE}==\"filesystem\", RUN{program}+=\"/usr/bin/systemd-mount --no-block --automount=yes --bind-device --options=fmask=111,dmask=000,noexec,nosuid,nodev,flush,sync --collect $devnode /run/mount/ext\"\n"
}
},
The device will be automatically mounted under /run/mount/ext.