Gets or sets the header text string for this notification.
A String value that is this notification's header text.
The Header property specifies the notification header, presented as bold text string, displayed above IToastNotificationProperties.Body and IToastNotificationProperties.Body2 regular strings. These text strings can occupy from one to three lines or be completely hidden depending on the currently applied IToastNotificationProperties.Template. See the Toast Notification Manager topic to learn more.