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>