Рахобрался сам:

На всякий случай, вдруг кто столкнется с той же проблемой:

private void KeyboardClick(object sender, EventArgs e)
        {
            MSG msg = new MSG();
            msg.hwnd = webBrowser1.Handle;
            msg.message = 258;
            msg.wParam = (System.IntPtr)116;
            msg.lParam = (System.IntPtr)0;
            IOleControl ax = (IOleControl)webBrowser1.ActiveXInstance;

            ax.OnMnemonic(ref msg);
        }

        [ComImport, Guid("B196B288-BAB4-101A-B69C-00AA00341D07"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
        public interface IOleControl
        {
            [PreserveSig]
            int GetControlInfo([Out] object pCI);
            [PreserveSig]
            int OnMnemonic([In] ref MSG pMsg);
            [PreserveSig]
            int OnAmbientPropertyChange(int dispID);
            [PreserveSig]
            int FreezeEvents(int bFreeze);
        }

        [Serializable, StructLayout(LayoutKind.Sequential)]
        public struct MSG
        {
            public IntPtr hwnd;
            public int message;
            public IntPtr wParam;
            public IntPtr lParam;
            public int time;
            public int pt_x;
            public int pt_y;
        }

Достаточно было описать интерфейсы