Monogame shader doesn't draw anything

Question

I'm brand new to working with shaders and I'm trying to learn how to use them with Monogame. I've been reading a few tutorials online about them and I'm already running into problems. It may be that the tutorials are outdated? When I apply the effect I just get a blank, cornflowerblue, screen.

#if OPENGL
#define SV_POSITION POSITION
#define VS_SHADERMODEL vs_3_0
#define PS_SHADERMODEL ps_3_0
#else
#define VS_SHADERMODEL vs_4_0_level_9_1
#define PS_SHADERMODEL ps_4_0_level_9_1
#endif

texture ScreenTexture;

sampler TextureSampler = sampler_state {
    Texture = <ScreenTexture>;
};

float4 PixelShaderFunction(float2 TextureCoordinate : TEXCOORD0) : COLOR0{
    float4 color = tex2D(TextureSampler, TextureCoordinate);
    return color;
}

technique Plain {
    pass Pass1 {
        PixelShader = compile PS_SHADERMODEL PixelShaderFunction();
    }
};

---

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace ShaderTest {
    /// <summary>
    /// This is the main type for your game.
    /// </summary>
    public class Game1 : Game {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        Effect shader1;
        Texture2D peach;

        public Game1() {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        /// <summary>
        /// Allows the game to perform any initialization it needs to before starting to run.
        /// This is where it can query for any required services and load any non-graphic
        /// related content.  Calling base.Initialize will enumerate through any components
        /// and initialize them as well.
        /// </summary>
        protected override void Initialize() {
            // TODO: Add your initialization logic here

            base.Initialize();
        }

        /// <summary>
        /// LoadContent will be called once per game and is the place to load
        /// all of your content.
        /// </summary>
        protected override void LoadContent() {
            // Create a new SpriteBatch, which can be used to draw textures.
            spriteBatch = new SpriteBatch(GraphicsDevice);

            shader1 = Content.Load<Effect>("shader1");
            peach = Content.Load<Texture2D>("peach");

        }

        /// <summary>
        /// UnloadContent will be called once per game and is the place to unload
        /// game-specific content.
        /// </summary>
        protected override void UnloadContent() {
            // TODO: Unload any non ContentManager content here
        }

        /// <summary>
        /// Allows the game to run logic such as updating the world,
        /// checking for collisions, gathering input, and playing audio.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Update(GameTime gameTime) {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();

            // TODO: Add your update logic here

            base.Update(gameTime);
        }

        /// <summary>
        /// This is called when the game should draw itself.
        /// </summary>
        /// <param name="gameTime">Provides a snapshot of timing values.</param>
        protected override void Draw(GameTime gameTime) {
            GraphicsDevice.Clear(Color.CornflowerBlue);

            spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend);
            shader1.CurrentTechnique.Passes[0].Apply();
            spriteBatch.Draw(peach, new Vector2(0, 0), Color.White);

            spriteBatch.End();
            base.Draw(gameTime);
        }
    }
}

without shader:

with shader: