Where to add compiler options like -ea in IntelliJ IDEA?

The -ea option to enable assertions is passed to the JVM not to the compiler. Add it as a VM option for your runtime configuration.

Specific location in the menu: Run > Edit Configurations... > Configuration > VM options:

You can also make IntelliJ IDEA support assertions by default by changing the default Application configuration:

In the top menu, press Run > Edit Configurations > Defaults > Application > VM options.

To enable the assertion feature for all application, you could just fill the VM options with -ea