If you want to add a new field when sc_layout is disabled, then, check this: Legacy:Adding a Field to the Service Catalog Checkout - ServiceNow Wiki.

This document is also written on the same lines, with Cart layouts enabled.

So, lets take an example of adding a Company field to the checkout below the Requested for field:

This field will then pass the value of that field to the Company field of the Service Catalog Request.

This example makes the following assumptions.

• This example is for an instance using two-step checkout with new cart layout v2. If two-step checkout is not enabled, enable it
• This example populates the Company field on the Service Catalog Request form. If the field does not appear on the form, configure the form

To add a field to the Service Catalog Checkout:

Go to System UI > UI Macros and create a new UI Macro named sc_cart_view_company.

In the XML editor, add the following code:

<?xml version="1.0" encoding="utf-8" ?>

/j:if

/j:jelly

Now, navigate to System UI > UI Pages and select the servicecatalog_checkout_one_v2 UI Page. Add this script to the Client script field:

function setCartValue() {

var newField = gel('core_company');

var myCart = gel('cart_id');

var cart_item = new GlideRecord('sc_cart_item');

cart_item.addQuery('cart', myCart.value);

cart_item.query();

if(cart_item.next()) {

cart_item.hints = "";

cart_item.update();

}

}

• For this example, the reference field was called core_company, and the field being populated on the request is company. If different fields are used:
  
  • Find this line: var company = gel('core_company');
    
  • In the line that starts with 'cart_item.hints' replace 'request.company' with the name of the field to be populated on the request ticket where 'request' is the request being generated and 'company' is the name of the field.
• Now navigate to Service Catalog > Maintain Cart Layouts and select Target type: Browser Screen > Cart Preview Screen (Two Step).
• Go to Widget Macros tab, Select Edit and add the UI Macro created "sc_cart_view_company" from the list collector.
• Update the Order(say, 650) of the created macro to decide its position relative to other macros.

If i missed something, or some corrections, any kinda queries are always welcome.