BottomLine_iOS.cs

private CALayer BottomLine;

...

private void TintCustomization(UITextField Control, MyEntry customEntry)
{

...

  BottomLine = new CALayer
  {
      BorderColor = customEntry.MyTintColor.ToCGColor(),
      BackgroundColor = customEntry.MyTintColor.ToCGColor(),
      Frame = new CGRect(0, Frame.Height / 2, Frame.Width / 4, 1f)
  };
  Control.Layer.AddSublayer(_line);
  
  ...
  
}