Starting from GNOME 3.26, the drawer showing 3rd party application icons was removed. As a consequence, it's no longer possible to display the !M icon in the system tray out-of-the-box.

This is not a NoMachine issue, but a behaviour introduced by GNOME 3, please see also:
https://blogs.gnome.org/aday/2017/08/31/status-icons-and-gnome/

In order to workaround the problem, it's possible to install the following GNOME extension:
https://extensions.gnome.org/extension/1031/topicons/

We suggest to consult the official documentation of your Linux for how to install it. Usually this extension needs to be enabled manually in the Extension Manager.

For example, on RHEL 9 we have used the following procedure:
1. sudo dnf install gnome-shell-extension-top-icons
2. logout
3. login
4. launch Extensions application (gnome-shell-extension-prefs)
5. enable Top Icons

As an alternative, you could choose to adopt a lightweight desktop which doesn't exhibits this behaviour, for example MATE or Xfce. Some instructions are available here: https://kb.nomachine.com/AR04K00667