If you want your students to fill out a intake form you created and have it pop up in a particular checkout page, you can do it when creating or editing the checkout page. Just go to:
Left Side Menu Bar > More Features > Checkout Pages
Under "Checkout Pages" page, scroll down to "General" section then click "Edit Checkout Page" button.
Under "Add / Edit Checkout Page" page, scroll down to "General Settings" until you find "When completed, form should they be taken to? (optional)" section.
On this section, just select the intake form you created and want to be filled out by the students that have purchased an item from that checkout page. That is how you would attach an intake form to a Checkout Page.
The student will see this on the thank you page. You can also choose to skip the thank you page and add the intake form link to the redirect URL section so that it goes straight to the intake form.
This could be used for waivers and surveys as well.