We are testing the preload fields inside our acquisition landing page, and we are able to make it through "?pkey="Primary Key" at the end of the url.
What we realized is that on the official documentation there is written
What does it refer to? To external landing pages not based on Adobe Campaign servers?
It's not possible to preload AC data outside AC(eg external website other than AC landing page).
The better way is to use AC form page as an iframe in external page. In this way, you can preload data in external page.