Simulating button click using Win32 API in .NET

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C#

WinForms

C#4.0

i want to invoke a button in another app but i can't to do it .. after searched i found only one API can do this .. it is SendMessage API but i can't to ues it ...
i develop an windows app that interact with Viber app ( social application like whatsapp ) . i want to pass number in a text box and click on chat logo to write message then sending it ... any help please

here is my sample code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using System.Windows.Forms;

namespace ViberMess
{
    public partial class Form1 : Form
    {

        // Get a handle to an application window.
        [DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
        public static extern IntPtr FindWindow(string lpClassName,
            string lpWindowName);

        // Activate an application window.
        [DllImport("USER32.DLL")]
        public static extern bool SetForegroundWindow(IntPtr hWnd);

        
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // Get a handle to the viber application. The window class 
            // and window name were obtained using the Spy++ tool.
            IntPtr calculatorHandle = FindWindow("Qt5QWindowIcon", "Viber +2xxxxxxxxx");

            // Verify that Viber is a running process. 
            if (calculatorHandle == IntPtr.Zero)
            {
                MessageBox.Show("Viber is not running.");
                return;
            }

            // Make Viber the foreground application and send it  
            // phone number

            SetForegroundWindow(calculatorHandle);
            //Send CTRL+d to open dialled windows
            SendKeys.SendWait("^d");
            
            //Pass phone number
            SendKeys.SendWait("+2010xxxxxxxx");
            //Here is i want to click on message logo then type text and press Enter
            //My problem how to click on message logo to type text
           
        }

        
    }
}