Prepopulate registration fields

Hello, I have a landing page which captures name, last, phone, zipcode, email and phone.
I then have a single product and add to cart after the person fills out that form.
My goal is to load those values into session(s) and when the user ADD TO CART, they will have those fields already filled out (the hika registration will grab the values from the session(s).

I was thinking I just need to locate the file that has the registration form, and in the Value put the session ID of the corresponding field.
Any suggestions or ideas on how I can get this working?

Hi,

It's the file "registration" of the view "user" that you can edit via the menu Display->views in order to set your default values in the fields.

I found it and able to populate Email, however I cannot find Firstname, lastname, zipcode, address, phone.

Where would I be able to find those?

These are custom fields.
They are generated dynamically as you can configure them via the menu Display->Custom fields.

You can set their default value like that:
$this->address->address_lastname = $lastname;
at the beginning of the registration file