How to use TaskId on BaseSession::RegisterTask?

It got it working now. As a matter of fact the registering of the task needs to happen inside the external application. That's where I got it wrong before. If you are interested see my example code.

The base application executing the task of the external application:

public class ExternalJLinkApplicationLauncher extends DefaultUICommandActionListener {
	@Override
	public void OnCommand() {
		try {
			JLinkApplication externalJLinkApp = pfcGlobal.GetProESession().StartJLinkApplication("externalJLinkApp",
					"com.testing.ExternalJLinkApplication", "start", "stop", "C:\\path\\to\\classfiles", "C:\\path\\to\\textfolder", true);

			String taskId = "externalJLinkApp001";

			Argument dummyArgument = pfcArgument.Argument_Create("dummyArgument", pfcArgument.CreateASCIIStringArgValue("value"));
			Arguments arguments = Arguments.create();
			arguments.append(dummyArgument);
			try {
				externalJLinkApp.ExecuteTask(taskId, arguments);
			} catch (XToolkitNotFound e) {
				System.out.println("Task could not be found in target application.");
			}

		} catch (jxthrowable jxt) {
			System.out.println(jxt.getMessage());
		}
	}

	class ExternalJLinkAppListener extends DefaultJLinkTaskListener {
		@Override
		public Arguments OnExecute(Arguments arg0) throws jxthrowable {
			System.out.println("externalJLinkApp started.");
			return arg0;
		}
	}
}

The external application:

public class ExternalJLinkApplication {
	private void registerTask() {
		try {
			String taskId = "externalJLinkApp001";
			try {
				pfcGlobal.GetProESession().RegisterTask(taskId, new ExternalJLinkAppListener());
			} catch (XJLinkTaskExists e) {
				System.out.println("Task " + e.GetTaskId() + " already exists.");
				return;
			}

		} catch (jxthrowable jxt) {
			System.out.println(jxt.getMessage());
		}

	}

	public static void start() {
		ExternalTestApp instance = new ExternalTestApp();
		instance.registerTask();
	}
	
	public static void stop() {
		
	}
	
	class ExternalJLinkAppListener extends DefaultJLinkTaskListener {
		@Override
		public Arguments OnExecute(Arguments arg0) throws jxthrowable {
			new JwPrintMessage().messageArea("TEST");
			System.out.println("ExternalJLinkApplication started.");
			return arg0;
		}
	}
}

Thank you all for your support and best regards!