Prompting user to complete a form during checkout

Hi,

Why not have the form directly inside the checkout ?
You can do that by creating custom fields of the table "order" via the menu Display>Custom fields.
These fields will be displayed by the "fields" view in the checkout workflow, and you can restrict them to only some products or categories of products.
Now if you really want to use a third party form page, you would need :
1. some code customization on the checkout to display a link to the form based on the products in the cart
.2 the development of a plugin to check that the form has been filled before validating the creation of the order
So it would require a developer to work on that for you.
That's why I would highly suggest to use HikaShop custom fields instead. The information will be directly linked to the order, you'll be able to to easily display the fields where you want with all the available settings and restrict the fields only for the products needing them, all that without any coding.