So I've been trying to open IE in Private using selenium (C#), this is the closest i've come so far:
InternetExplorerOptions op = new InternetExplorerOptions();
op.PageLoadStrategy = PageLoadStrategy.Normal;
op.IgnoreZoomLevel = true;
op.InitialBrowserUrl = "https://entry.wgrintra.net/schadenwv/servlet/main";
op.ForceCreateProcessApi = true;
op.BrowserCommandLineArguments = "-private";
IWebDriver driver = new InternetExplorerDriver(op);
The problem here is, after 60 seconds of opening the browser (correctly in private) the driver times out (last step doesn't finish).
I've looked around a lot, most is just using Capabilities which are not useful anymore.
(I had to add a value to the registry to be able to forcecreate the process api)
Try to refer code example below and make a test with it on your side may help to solve your issue.
Reference:
How to Open Internet Explorer Browser in Incognito / Private mode using Selenium / WebDriver ?