Is there a Way to fill a already opened Webpage in C#?

I have to write a Programm that saves data from a RS232 connection (done) and writes this data into a form on a Webpage (TODO). The user has this webpage already opened on his Display. How can I access this already opened Webpage and fill out the Form on it?

After searching for a while I found the Webbrowser Class, but it didnt look like i could do would i need it to do...

I hope you can help me!

Thanks in advance!