How to use PL/SQL blocks in SQL statements?

I'm trying to run SQL statements which contain PL/SQL blocks, e.g.:

DECLARE
id_test NUMBER(11);
BEGIN

SELECT SEQ$TEST$ID.nextval
INTO id_test
FROM dual;

INSERT INTO TEST ("ID", "APPLICATION")
VALUES (id_test, 'NeoLoad');

END;
/

The SQL has been successfully tested with PL/SQL Developer and sqlplus, however Check User Path fails with the following error:

<Output>
<Status state="ERROR">An error:
java.sql.SQLException: ORA-06550: line 2, column 20:
PLS-00103: Encountered the symbol "end-of-file" when expecting one of the following:

:= ; not null default character
This occurred while executing query:
DECLARE
id_test NUMBER(11)</Status>
</Output>

Did I miss smth or are PL/SQL blocks not supported?