How to choose a file in an already opened FileDialog

I want to choose a file from an already opened OpenFileDialog which is shown by the browser.

I want to use code similar to the following example:

OpenFileDialog dialog = new OpenFileDialog();
dialog.Filter = "txt files (*.txt)|*.txt|All files (*.*)|*.*";
dialog.InitialDirectory = "C:\\";

The problem is, that I cant access dialog. I can't reference it.