Create a stored procedure class that is available on the H2 database engine classpath:

public class H2StoredProcedures {
    public static String stringIn(final String inValue) {
        System.out.println("inValue: '" + inValue + "'");
        return "some_result";
    }
}

Create and alias in your schema for the stored procedure class:

DROP ALIAS IF EXISTS STRING_IN;
CREATE ALIAS STRING_IN FOR "H2StoredProcedures.stringIn";

Call the stored procedure like any other except without an "out" parameter:

CALL STRING_IN('fooIn');

Those parts are pretty straight forward so I'm guessing you're probably asking how the results are returned from the driver implementation?