fill javascript textboxes using webform

Question

I am trying to write a winforms application that will automatically fill in textboxes on webpages.  The problem is that the textboxes are part of a javascript function that is on a separate, unavailable website.  Is there any way to create a winform application with two corresponding textboxes and a button that when clicked will automatically navigate to the webpage and fill-in the boxes in the js function?  I can get the button to navigate to the webpage using .NET's webbrowser control, but I can't progress farther than that.

Thanks