How to fill in web form fields

Hiya guys!

I'm new to this forum, and new to C#, and have hit upon a snag in a project i'm working on.

I am trying to load an existing webpage into .net, and populate the fields on this page.
So far, I have created a new web form, and I can grab data from the DOM.

I have found the fieldnames that I want to populate, but I have no idea how to populate them!
It looks like the webpage is read only, as I cannot set any properties.

Can someone please point me in the right direction?
Sorry if its a simple question, but I have only started learning c# this week.

Thanks in advance!
DB