It's possible, but not with notify-send. You need to talk to your desktop notifications service with a less specialized tool, like gdbus. It will allow you to call various methods from a shell.
An alternative is dbus-send, but it doesn't (at least for now) support the variant type needed by the org.freedesktop.Notifications.Notify method. We will stick to gdbus.
Check man 1 gdbus. It includes an example which sends a notification:
gdbus call --session \
--dest org.freedesktop.Notifications \
--object-path /org/freedesktop/Notifications \
--method org.freedesktop.Notifications.Notify \
my_app_name \
42 \
gtk-dialog-info \
"The Summary" \
"Here's the body of the notification" \
[] \
{} \
5000
This should print (uint32 42,) where 42 is the notification ID. In fact the example specifies 42 explicitly, which means if there already is a notification with this exact ID then it will be replaced. To create a brand new notification specify 0. A hardcoded ID (like 999 in your question) is possible, but what if the ID is already taken by some unrelated, possibly important notification? For this reason specify 0, capture the output, isolate the new ID and use it with org.freedesktop.Notifications.CloseNotification.
Useful links:
Example script:
#!/bin/sh
tty="$(tty)"
id="$(gdbus call --session \
--dest org.freedesktop.Notifications \
--object-path /org/freedesktop/Notifications \
--method org.freedesktop.Notifications.Notify \
my_script \
0 \
utilities-terminal \
"Response required" \
"Script awaiting input ($tty)." \
[] \
{} \
0
)"
id="${id##* }"
id="${id%,)}"
printf 'Type here: '
read foo
>/dev/null gdbus call --session \
--dest org.freedesktop.Notifications \
--object-path /org/freedesktop/Notifications \
--method org.freedesktop.Notifications.CloseNotification \
"$id"
