using System;
using Xamarin.Forms;

namespace MyApp.Controls
{
    public class CustomFrame : Frame
    {
        // ---------------------------------------------------------------------------------------------------------------
        public static readonly BindableProperty CornerRadiusTopLeftProperty = BindableProperty.Create(
            propertyName: "CornerRadiusTopLeft",
            returnType: typeof(bool),
            declaringType: typeof(CustomFrame),
            defaultValue: true,
            defaultBindingMode: BindingMode.TwoWay
        );

        public bool CornerRadiusTopLeft
        {
            get { return (bool)GetValue(CornerRadiusTopLeftProperty); }
            set { base.SetValue(CornerRadiusTopLeftProperty, value); }
        }

        // ---------------------------------------------------------------------------------------------------------------
        public static readonly BindableProperty CornerRadiusTopRightProperty = BindableProperty.Create(
            propertyName: "CornerRadiusTopRight",
            returnType: typeof(bool),
            declaringType: typeof(CustomFrame),
            defaultValue: true,
            defaultBindingMode: BindingMode.TwoWay
        );

        public bool CornerRadiusTopRight
        {
            get { return (bool)GetValue(CornerRadiusTopRightProperty); }
            set { base.SetValue(CornerRadiusTopRightProperty, value); }
        }

        // ---------------------------------------------------------------------------------------------------------------
        public static readonly BindableProperty CornerRadiusBottomLeftProperty = BindableProperty.Create(
            propertyName: "CornerRadiusBottomLeft",
            returnType: typeof(bool),
            declaringType: typeof(CustomFrame),
            defaultValue: true,
            defaultBindingMode: BindingMode.TwoWay
        );

        public bool CornerRadiusBottomLeft
        {
            get { return (bool)GetValue(CornerRadiusBottomLeftProperty); }
            set { base.SetValue(CornerRadiusBottomLeftProperty, value); }
        }

        // ---------------------------------------------------------------------------------------------------------------
        public static readonly BindableProperty CornerRadiusBottomRightProperty = BindableProperty.Create(
            propertyName: "CornerRadiusBottomRight",
            returnType: typeof(bool),
            declaringType: typeof(CustomFrame),
            defaultValue: true,
            defaultBindingMode: BindingMode.TwoWay
        );

        public bool CornerRadiusBottomRight
        {
            get { return (bool)GetValue(CornerRadiusBottomRightProperty); }
            set { base.SetValue(CornerRadiusBottomRightProperty, value); }
        }
    }
}

using System;

using CoreAnimation;

using MyApp.iOS.CustomRenderers;
using Foundation;
using MyApp.Controls;
using UIKit;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer(typeof(CustomFrame), typeof(CustomFrameRenderer))]
namespace MyApp.iOS.CustomRenderers
{
    public class CustomFrameRenderer : FrameRenderer
    {
        protected override void OnElementChanged(ElementChangedEventArgs<Frame> e)
        {
            base.OnElementChanged(e);

            if (Element != null)
            {
                var element = Element as CustomFrame;

                int result = 0;

                if (element.CornerRadiusTopLeft)
                    result += (int)CACornerMask.MinXMinYCorner;

                if (element.CornerRadiusTopRight)
                    result += (int)CACornerMask.MaxXMinYCorner;

                if (element.CornerRadiusBottomLeft)
                    result += (int)CACornerMask.MinXMaxYCorner;

                if (element.CornerRadiusBottomRight)
                    result += (int)CACornerMask.MaxXMaxYCorner;

                Layer.MaskedCorners = (CACornerMask)result;
            };
        }
    }
}

xmlns:customControls="clr-namespace:MyApp.Controls"

<customControls:CustomFrame BackgroundColor="White" CornerRadius="10" HasShadow="false"
        CornerRadiusBottomLeft="false" CornerRadiusBottomRight="false" VerticalOptions="Start" HorizontalOptions="FillAndExpand" Padding="15">

    <!-- Add your other elements here -->
</customControls:CustomFrame>