How can I retrieve data (changes constantly) from Web after login ?

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C#

HTML

web

Please I want to buil a small app (C#) that gets data from a website, but firstly I need to login. Please if you can help my, beacuse I ran all over the internet and I can't clearly understand because I am only 16 years old.

What I want to do is to take the data and display it in a textBox. But:
- the site firstly requires login
- (after login) the data is displayed only in DOM (inspect element) and not in the source code
- the data changes constantly

I know that I need to use:
- HAP (Html Agility Pack)
- webClient not webBrowser (because I want only the data from the page, not to display the hole page)

The site: https://www.trading212.com/en/login/[^]
After loging the data: http://i.stack.imgur.com/1dz8Q.png[^]
<Removed username and password>
Maybe for you this is a simple task but for my is quite hard to understand, so if anybody has time and could help me I will be very grateful!