magento2 – Adding a new field to one page checkout

I have a special field (Customer reference no, field id = cus_ref_no) that i need to add to checkout.
it is not related to shipping and billing. I have been following the answer from here, but for adding the custom field to checkout page app/code/Vendor/Module/view/frontend/layout/checkout_index_index.xml
I am not sure how to do this part – since the field is neither part of shipping and billing. It is an independent field that needs to be displayed as the first step on the checkout. How should i proceed to adding this field?

And how can i sort the field so that it is displayed as the first field on the checkout page – since i do not see this in the answer to the previous question?