`java.lang.NoClassDefFoundError` from javaagent built by `maven package` - maven

Question

I've been trying to build javaagent (containing premain()) to .jar file using maven mvn package, and keep getting java.lang.NoClassDefFoundError exception related to external dependencies. For details, I added kafka-client dependecies and maven-jar-plugin to pom.xml like below
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<configuration>
<archive>
<index>true</index>
<manifest>
<addClasspath>true</addClasspath>
<classpathPrefix>lib</classpathPrefix>
</manifest>
<manifestEntries>
<mode>development</mode>
<url>${project.url}</url>
<key>value</key>
<Premain-Class>com.my.java.monitoring.Agent</Premain-Class>
<Can-Redefine-Classes>true</Can-Redefine-Classes>
<Can-Retransform-Classes>true</Can-Retransform-Classes>
</manifestEntries>
</archive>
</configuration>
</plugin>
After reading How can I create an executable JAR with dependencies using Maven?, I tried using maven-assembly-plugin instead, like below
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
<archive>
<index>true</index>
<manifest>
<addClasspath>true</addClasspath>
</manifest>
<manifestEntries>
<Premain-Class>com.my.java.monitoring.Agent</Premain-Class>
</manifestEntries>
</archive>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<!-- this is used for inheritance merges -->
<phase>package</phase>
<!-- append to the packaging phase. -->
<goals>
<goal>single</goal>
<!-- goals == mojos -->
</goals>
</execution>
</executions>
</plugin>
but I get this error =>
Failed to find Premain-Class manifest attribute in target/my-java-agent.jar Error occurred during initialization of VM agent library failed to init: instrument
Can anyone who has experience building javaagent .jar file with dependencies, please help.