Introducing hidden fields during runtime

I want to create associate a custom Post Type (as parent) to every single post( from USP forms).
I am looking for an approach that will pass on the parent post ID when the USP form is submitted.

Here are the approaches I am having difficulty with:
1) Using one of the USP hooks that will insert a hidden input field
2) Creating Custom fields that can be introduced as hidden dynamically during render time of every single form.

Please let me know what approach to use, and an sample code snippet to guide me?
Thank you