To simulate the auto form-fill functionality of a website in your application using HTML Agility Pack (HAP), you must handle the dynamic behavior that websites typically execute through JavaScript. Since HAP doesn't execute JavaScript, you will need to reverse-engineer how the form is populated and reproduce that logic in your code.

This involves:

1. Analyzing the Website's Behavior
2. Manipulating the HTML
3. Simulating Script Logic
4. Posting the Data