Help! Adding UI elements to Desk

Hello, I am following the official doc for ERPNext and Frappe, and am trying to add an input field to Desk using Client Script.

Following this article List I see how to add a button to Desk. I then studied public/js/frappe/ui/page.js, and found the add_data() function to add an INPUT to Desk. Great!

Now I need to reference the value in this INPUT when I click the button. As all elements in Desk page doesn’t have ID, and there will be multiple input-with-feedback, input-with-, I wonder how to reference a particular input-with-?

Also, is it possible to adjust the position of my new INPUT?

Thank you very very much