How do I pass a value from a field to the A+ form?

Hi,

We want to collect an Inventory number in a field on one page, before scheduling the appointment on another page where that inventory number would be plugged into one of the additional fields.

1. How do we transfer the data from one field to an additional field?

2. Can we hide that additinal field? how?

3. To create the first field that would collect and transfer the inventory number, would it be easier to use Gravity Forms, or build a custom field/form?

Thank you