Directus has a number of extensions.

In the Dashboard, extensions are installed in the extensions folder. This is set to /app/data/extensions.

We have a helper script at /app/pkg/install-extension.sh to install extensions:
/app/pkg/install-extension.sh directus-extension-wpslug-interface
- added 1 package in 1s
- Restart Directus to enable the extension

Info about a possible notification:
If the script above ends with Killed, it means that it got killed by the out of memory killer. To fix this, increase the memory limit of the app to 2GB (or more) and re-run the script.