I need to obtain the post_id of a post created by a form, in this case from the last entry submitted by the current user. I tried to do this by creating a hidden field with [post_id] as the default value. Then referencing it with

[frm-field-value field_id=x user_id=current]

where x is the id of the hidden field containing [post_id]. But it doesn't work: the [post_id] fields remain empty within the submitted entries, although the [post_id] shortcode works fine in the Views.
This seems too simple to need coding. What is the intended way to do it?