我测试了代码的情况下提供的答案，它的访问器丢失：Get和Set方法：

public class KeyboardStyle
{
    public static BindableProperty KeyboardFlagsProperty = BindableProperty.CreateAttached
    (
        propertyName: "KeyboardFlags", 
        returnType: typeof(string), 
        declaringType: typeof(InputView), 
        defaultValue: null, 
        defaultBindingMode: BindingMode.OneWay, 
        propertyChanged: HandleKeyboardFlagsChanged
    );
    public static string GetKeyboardFlags(InputView view)
    {
        return (string)view.GetValue(KeyboardFlagsProperty);
    }
    public static void SetKeyboardFlags(InputView view, string value)
    {
        view.SetValue(KeyboardFlagsProperty, value);
    }

    ...

}

然后您可以在App. xaml中使用它：

<Application xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:forms" 
             x:Class="forms.App">
    <Application.Resources>
        <Style TargetType="Entry">
            <Setter Property="local:KeyboardStyle.KeyboardFlags" 
                    Value="Spellcheck,CapitalizeSentence"/>
        </Style>
    </Application.Resources>
</Application>

效果很好。
详细信息请参考官方创建附加属性的访问器。