JPA @Index how to use part of column value?

Hibernate ORM
1

Say I have columns foo varchar(255), bar varchar(255), baz varchar(1000) … with JPA how would I define a compound index (@Index) that included all of foo and bar, but only the first 255 chars of baz?

… to generate for MySQL:

…
KEY myindex (foo,bar,baz(255))
…