Sorry, I didn’t understood you before; you mean the post_id of the post that acf creates? That makes sense; but I have different versions of the db for each server so the ID is not the same for all, I guess I could make a query to find it out but that would be more work for the same result.
I’m creating the post at the same time (using wp_insert_post) so ‘_field_name’ doesn’t exist quite yet.