From 7.2 onwards you can do the following to preload forms via post:

- Setup a simple form in a HTML snippet that redirects to the page with the form you wish to preload on.

- The form in your HTML snippet just needs to have a hidden input "preload_from_post" plus any form field names and values as the other hidden inputs. (Where the input names are the same as the input names on the form you wish to preload.) 

e.g. with a snippet like this:

Tt will per-load your form like this: