Lines from all sides looks enough strange on iPhone X +.
Typically there is a top line only.
To make this line you can retrieve tabBar and to set shadowImage property.
Looks simple, but you need to creare a image (not very comfortable).
IMO, more simple is to add a subview. For example:
Dim noTabControl As NativeObject = TabControl
Dim noTabBar As NativeObject = noTabControl.GetField ("tabBar")
Dim View As View = noTabBar
Dim Panel As Panel
Panel.Width = View.Width
Panel.Height = 1
Panel.Color = Colors.Red
noTabBar.RunMethod ("addSubview:", Array (Panel))