Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using CommandLine;

namespace sendkeys
{
    class Program
    {
        [DllImport("User32.dll")]
        static extern int SetForegroundWindow(IntPtr point);

        static void Main(string[] args)
        {
            CommandLine.Parser.Default.ParseArguments<Options>(args)
              .WithParsed(RunOptions)
              .WithNotParsed(HandleParseError);
#if DEBUG
            Console.ReadLine();
#endif
        }

        private static void HandleParseError(IEnumerable<Error> errs)
        {
        }

        private static void RunOptions(Options options)
        {
            if(options.KeyHelp)
            {
                KeyHelp();
                return;
            }

#if DEBUG
            Console.WriteLine("Targeted window: {0}", options.WindowName);
            Console.WriteLine("Key to be sent: {0}", options.Key);
#endif
            if (options.WindowName == "" || options.Key == "")
                return;

            try
            {
                Process[] proc = Process.GetProcesses();
                foreach (Process p in proc)
                {
                    if (p.MainWindowTitle==options.WindowName)
                    {
                        IntPtr h = p.MainWindowHandle;
                        SetForegroundWindow(h);
                        SendKeys.SendWait(options.Key);
                        break;
                    }
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex);
            }
        }

        /// <summary>
        /// Extended help for keys. 
        /// </summary>
        private static void KeyHelp()
        {
            // Source: https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.sendkeys.send?view=netcore-3.1

            string keyHelp;
            keyHelp =
@"Each key is represented by one or more characters. To specify
a single keyboard character, use the character itself.
For example, to represent the letter A, pass in the string 'A'
to the method. To represent more than one character, append
each additional character to the one preceding it.
To represent the letters A, B, and C, specify the parameter as 'ABC'.

The plus sign (+), caret (^), percent sign (%), tilde (~), and
parentheses () have special meanings. To specify one of these characters,
enclose it within braces ({}). For example, to specify the plus sign, use {+}. 
To specify brace characters, use {{} and {}}. Brackets ([ ]) have no 
special meaning to SendKeys, but you must enclose them in braces. 
In other applications, brackets do have a special meaning that 
might be significant when dynamic data exchange (DDE) occurs.

To specify characters that aren't displayed when you press a key, 
such as ENTER or TAB, and keys that represent actions rather than 
characters, use the codes in the following table.

Key              Code
BACKSPACE        {BACKSPACE}, {BS}, or {BKSP}
BREAK            {BREAK}
CAPS LOCK        {CAPSLOCK}
DEL or DELETE    {DELETE} or {DEL}
DOWN ARROW       {DOWN}
END              {END}
ENTER            {ENTER}or ~
ESC              {ESC}
HELP             {HELP}
HOME             {HOME}
INS or INSERT    {INSERT} or {INS}
LEFT ARROW       {LEFT}
NUM LOCK         {NUMLOCK}
PAGE DOWN        {PGDN}
PAGE UP          {PGUP}
PRINT SCREEN     {PRTSC} (reserved for future use)
RIGHT ARROW      {RIGHT}
SCROLL LOCK      {SCROLLLOCK}
TAB              {TAB}
UP ARROW         {UP}
F1               {F1}
F2               {F2}
F3               {F3}
F4               {F4}
F5               {F5}
F6               {F6}
F7               {F7}
F8               {F8}
F9               {F9}
F10              {F10}
F11              {F11}
F12              {F12}
F13              {F13}
F14              {F14}
F15              {F15}
F16              {F16}
Keypad add       {ADD}
Keypad subtract  {SUBTRACT}
Keypad multiply  {MULTIPLY}
Keypad divide    {DIVIDE}

To specify keys combined with any combination of the SHIFT, CTRL, and ALT keys,
precede the key code with one or more of the following codes.

Key	             Code
SHIFT            +
CTRL             ^
ALT              %

To specify that any combination of SHIFT, CTRL, and ALT should be held down 
while several other keys are pressed, enclose the code for those keys in 
parentheses. For example, to specify to hold down SHIFT while E and C are 
pressed, use +(EC). To specify to hold down SHIFT while E is pressed, followed 
by C without SHIFT, use +EC.

To specify repeating keys, use the form {key number}. You must put a space 
between key and number.For example, {LEFT 42} means press the LEFT ARROW 
key 42 times; {h 10} means press H 10 times.";
            
            Console.WriteLine(keyHelp);
        }

        public class Options
        {
            [Option('w',
                Required = false,
                HelpText = "Window name where the key will be sent.")]
            public string WindowName { get; set; }

            [Option('k',
                Required = false,
                HelpText = "The key to be sent.")]
            public string Key { get; set; }

            [Option("keyhelp",
                Required = false,
                HelpText = "Display extented help")]
            public bool KeyHelp { get; set; }
        }
    }
}