draw.io is an app that helps you create diagrams. There are different ways to access the app, but I will be talking about the desktop app specifically.

Sometimes the default functionality isn't enough, and so there is a mechanism to add more abilities through plugins.

Plugins can be accessed through the Extras>Plugins... menu. The modal that pops up shows the plugins that are enabled.

If you want to add more plugins you can select the Add button and pick from some built-in plugins.

By default external (custom) plugins are not allowed. To allow external plugins you need to start the app with an option that enables them. Here is an example of the command I used in Windows:

 bash
./draw.io.exe --enable-plugins

After starting the app again, going to the plugin modal, and clicking the Add button; the ui should have a drop down to select a local plugin file.

Here is the release that disabled the external plugins by default.