Multi page form needs dynamic content based on starting page drop down selection

I have a multi page form that needs to grab some dynamic content.

Here is my scenario:
Page 1 -
drop down field.
Page 2 -
query dynamic content based on page 1 drop down field.
populate field value with dynamic content.
Page 3 -
Submit form

I can get the dynamic content to populate the field value with no issues but I can't figure out how to pass the drop down field selection to my script when the user selects "next" on the form.

Okay, so use ajax to run my php script then pass the result back to the field value... do you have any code examples?