Linking between Created Posts and Form Entires

I started a site with Formidable so I don’t know if this is possible or if I need to be displaying my data from form entries rather than posts but this is what I have:

I have posts created using a Formidable form, and I display this data in a custom template. What I’d like to do is allow users to edit the posts using Formidable. What I’ve done is added a custom field to the post that contains the ID of the form entry so I can add the shortcodes to edit the entry and that works but what I want to do is have that custom field populated when I create the post using the form and I can’t seem to  work that out.

Is this achievable or is there a better way to relate a created post with a form entry.