Service cloud snap-ins pre-chat form

I am using snap-in with pre-chat enabled to implement chat functionality on our website. I am using the code snippet that is generated by the snap-in. Everything works great but I have 2 questions:

1. I want to hard code the field “origin” of the case object in the pre-chat code. I am not sure how to insert that into the pre-generated code snippet. I have copied the code snippet below.

2. The pre-chat form has from the snap-in has only limited number of fields available out of the box. For example, if I have a custom field on the case object, that custom field isn’t available on the pre-chat form. How can i make custom fields available?