This is the de facto example for har packaging. This example pom.xml demonstrates what you need in order to have maven generate a HAR file using maven-har-plugin. The <extensions/> element must be present and must be set to true for the plugin to operate correctly. Also remember to set <packaging/> to har.
<project>
...
<build>
...
<plugins>
...
<plugin>
<groupId>net.sf.maven-har</groupId>
<artifactId>maven-har-plugin</artifactId>
<version>1.0</version>
<extensions>true</extensions>
</plugin>
...
</plugins>
...
</build>
...
<packaging>har</packaging>
...
</project>