You need to import this function like this:
using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
process = Process.Start(@"D:\Games\SteamLibary\steamapps\common\ARK\ShooterGame\Binaries\Win64\Shootergame.exe");
handle = process.MainWindowHandle;
SetForegroundWindow(handle);
[DllImport("user32.dll")]
private static extern bool SetForegroundWindow(IntPtr hWnd);