You can't declare an extern local method inside of a method, or any other method with an attribute. Move your DLL import into the class:
using System.Runtime.InteropServices;
public class WindowHandling
{
[DllImport("User32.dll")]
public static extern int SetForegroundWindow(IntPtr point);
public void ActivateTargetApplication(string processName, List<string> barcodesList)
{
Process p = Process.Start("notepad++.exe");
p.WaitForInputIdle();
IntPtr h = p.MainWindowHandle;
SetForegroundWindow(h);
SendKeys.SendWait("k");
IntPtr processFoundWindow = p.MainWindowHandle;
}
}