A workaround for multisampling in MonoGame under UWP by rendering to texture first.

MainView.cs

/*
One cannot use multisampling directly in a UWP application because
of reasons stipulated here: 
https://docs.microsoft.com/tr-tr/windows/uwp/gaming/multisampling--multi-sample-anti-aliasing--in-windows-store-apps
The workaround is to create a render target with multisampling, render to that
and then render the render target as a sprite
*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Transactions;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace Wut
{
    public class MainView : Game
    {
        private readonly GraphicsDeviceManager graphics;

        private SpriteFont defaultFont;
        private SpriteBatch spriteBatch;
        private RenderTarget2D renderTarget;

        public MainView()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {
            base.Initialize();
        }

        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);
            defaultFont = Content.Load<SpriteFont>("Default");
        }

        protected override void Update(GameTime gameTime)
        {
            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            UpdateRenderTarget();
            DrawInternal(gameTime);
            ApplyRenderTarget();
            base.Draw(gameTime);
        }

        private void DrawInternal(GameTime gameTime)
        {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            spriteBatch.Begin();
            spriteBatch.DrawString(defaultFont, "Hello World", new Vector2(10, 10), Color.White);
            spriteBatch.DrawCircle(new Vector2(graphics.PreferredBackBufferWidth / 2.0f, graphics.PreferredBackBufferHeight / 2.0f), 200, 32, Color.Green);
            spriteBatch.End();
        }

        private void UpdateRenderTarget()
        {
            bool change = renderTarget == null || 
                renderTarget.Width != graphics.PreferredBackBufferWidth ||
                renderTarget.Height != graphics.PreferredBackBufferHeight;
            if (change)
            {
                renderTarget?.Dispose();

                renderTarget = new RenderTarget2D(
                    GraphicsDevice,
                    graphics.PreferredBackBufferWidth,
                    graphics.PreferredBackBufferHeight,
                    false,
                    graphics.PreferredBackBufferFormat,
                    graphics.PreferredDepthStencilFormat,
                    8,
                    RenderTargetUsage.PlatformContents);
            }
            GraphicsDevice.SetRenderTarget(renderTarget);
        }

        private void ApplyRenderTarget()
        {
            GraphicsDevice.SetRenderTarget(null);

            spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.Opaque,
                SamplerState.LinearClamp, DepthStencilState.Default,
                RasterizerState.CullNone);

            spriteBatch.Draw(renderTarget, new Rectangle(0, 0, renderTarget.Width, renderTarget.Height), Color.White);

            spriteBatch.End();
        }
    }
}

The DrawCircle thing is from here