Note: Advice in this article will only work for DotNetBrowser 1.
See the corresponding article for DotNetBrowser 2 here.

The sandbox is a technology that provides an ability to launch an executable native process within a restricted environment. All permissions of the process are explicitly defined by the specified policies.

Chromium engine supports sandbox. By default, sandbox in the Chromium engine used by DotNetBrowser is disabled. To enable sandbox set the BrowserPreferences.SandboxEnabled property to true. This property must be specified before creating any BrowserView instance.

When the Chromium sandbox is disabled, the PPAPI Flash player plugin briefly displays a console window during startup. This console window displays "NOT SANDBOXED" text and disappears immediately. The only way to prevent this console window from showing is to enable the Chromium sandbox.