[c#] How to use [DllImport("")] in C#?

I found a lot of questions about it, but no one explains how I can use this.

I have this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using Microsoft.FSharp.Linq.RuntimeHelpers;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.IO;

public class WindowHandling
{
    public void ActivateTargetApplication(string processName, List<string> barcodesList)
    {
        [DllImport("User32.dll")]
        public static extern int SetForegroundWindow(IntPtr point);
        Process p = Process.Start("notepad++.exe");
        p.WaitForInputIdle();
        IntPtr h = p.MainWindowHandle;
        SetForegroundWindow(h);
        SendKeys.SendWait("k");
        IntPtr processFoundWindow = p.MainWindowHandle;
    }
}

Can someone help me to understand why it gives me an error on the DllImport line and on the public static line?

Does anyone have an idea, what can I do? Thank you.