[web2py] How to add form vars to crud.create onaccept?

[Tom Atkins]

I have something like this which works fine:

    commentform = SQLFORM(db.comment)
            commentform.vars.post_id = post.id

This populates commentform with the current post id.

How can I do this with crud.create? (I know I need to use the onaccept
method of crud.create but I'm stuck.)