What's the right way to add directories to XDG_DATA_DIRS?

I've installed some app under /opt/myapp, which has a /opt/myapp/share directory. As I finish installing, it tells me: Note that '/opt/myapp/share' is not in the search path set by the XDG_DATA_HOME and XDG_DATA_DIRS environment variables, so applications may not be able to find it until you set them. The directories currently searched are: - /usr/share/gnome - /home/joeuser/.local/share/flatpak/exports/share - /var/lib/flatpak/exports/share - /usr/local/share - /usr/share What's the right way to add directories to that list - system-wide and as a single user?