How do you create a custom xaml property?

I'm creating a custom Entry component by wrapping a Entry around a StackLayout. Currently I'm trying to create a property called 'SandboxPlaceholder' and I simple want to redirect this property to the Entry component's 'Placeholder' property.

I'm trying something like this, but with no success. Please help guide me in the right direction.

Edit: My current workaround is setting the properties programmatically, but if I can figure how to achieve it using xaml, my development will be supercharged.

<unicareer:ViewEntrySettings
    x:Name="EntryEmail"
    SandboxPlaceholder="Hello"/>



public class ViewEntrySettings : StackLayout {

    Entry entry;

    public ViewEntrySettings () {

        UtilLogger.Log ("ViewEntrySettings", "Constructor");

        Orientation = StackOrientation.Horizontal;
        HorizontalOptions = LayoutOptions.FillAndExpand;
        HeightRequest = 40;
        Padding = new Thickness (6, 0, 6, 0);
        BackgroundColor = Color.White;

        entry = new Entry {
            Keyboard = Keyboard.Email,
            Placeholder = SandboxPlaceholder,
            HorizontalOptions = LayoutOptions.FillAndExpand,
        };

        Children.Add (entry);

        UtilLogger.Log("ViewEntryLogin_Uni", "Sandbox: " + SandboxPlaceholder);
    }


    public static BindableProperty SandboxPlaceholderProperty = BindableProperty.Create<ViewEntrySettings, string>(p => p.SandboxPlaceholder, "Default");
    public string SandboxPlaceholder { 
        get
        {
            UtilLogger.Log ("ViewEntrySettings", "SandboxProperty get");
            return (string) GetValue(SandboxPlaceholderProperty);
        } 
        set {
            UtilLogger.Log ("ViewEntrySettings", "SandboxProperty set");
            SetValue(SandboxPlaceholderProperty, value);
            if(entry != null) {
                    entry.Placeholder = value;}
        } 
    }
}