To conveniently extend the functionality of adding shipment, it would be nice to add model transfer to a widget in a zone AdminWidgetZones.OrderShipmentAddButtons.
In Nop.Web\Areas\Admin\Views\Order\AddShipment.cshtml
Change
@await Component.InvokeAsync("AdminWidget", new { widgetZone = AdminWidgetZones.OrderShipmentAddButtons })
to
@await Component.InvokeAsync("AdminWidget", new { widgetZone = AdminWidgetZones.OrderShipmentAddButtons, additionalData = Model })