[@fterra Participant 10 years, 7 months ago]

Yes, @antipole,
after I wrote to you I tried the bbp_forum_attributes_metabox_save hook and it has worked for me.
But I guess it’s also possible to use wp_insert_post hook as suggested by John James Jacoby.
bbp_forum_attributes_metabox_save will most likely fire only in admin page.
On the other hand wp_insert_post will most likely fire both in admin page and front-end. Seems like front-end forum creation requires the use of [bbp-forum-form] shortcode, which I have never tried.