Just run, the below as the content of your script step:
String headerOne = "abc";
String headerTwo = "def";
String finalOutput = headerOne + headerTwo;
System.out.println(finalOutput);
As the name of the step says, it runs the script that is stored in the step. For Beanshell this script can contain Java statements.
You can think of it like the script step run the content of some main() method. The step does NOT run itself the main() method of some class.
If you want to use your class then you need to put your compiled class in a jar file and add that jar to the HotdDeploy directory such that DevTest can load the class. Then your script can execute the statement:
trial.main();
If you want to store your string concatenation for later use in your testcase and/or vsm then you can add:
testExec.setStateValue("finalOutput", finalOutput);
Now, after the script step has executed a property will be available within the environment which you can use in other steps as {{finalOutput}}
Hope this helps,
Danny