First of all HTMLAgilityPack it's not for simulating user interaction in a web page, HTMLAgilityPack is an agile HTML parser that builds a read/write DOM and supports plain XPATH or XSLT (you actually don't HAVE to understand XPATH nor XSLT to use it, don't worry...).
HTMLAgilityPack not support JavaScript, it's a very important step, because many developers get trouble with the full load of the page in the browser and the request made by HTMLAgilityPack or any library you use to make the request.
For user interaction, full load of the web page, web testing I strongly recommend you Selenium, Selenium automates browsers. Selenium has support for several programming languages (Java, C#, Ruby, Python, etc), you can read more in the above link with a very good documentation.
The only drawback of Selenium is its open a browser to make the work, but it can be simulated in some environments to run headless browser, you can read more about this in the following links :
I hope this help you