CreatePageTemplate.cs

using Microsoft.SharePoint.Client;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Security;
using System.Text;
using System.Threading.Tasks;
using ClientSidePage = OfficeDevPnP.Core.Pages.ClientSidePage;

namespace ConsoleApp1
{
    class p3
    {
        static void Main(string[] args)
        {
            string userName = "username";
            string password = "password";
            using (ClientContext clientContext = new ClientContext("https://<your-site-url>"))
            {
                SecureString securePassword = new SecureString();
                foreach (char c in password.ToCharArray())
                {
                    securePassword.AppendChar(c);
                }

                clientContext.AuthenticationMode = ClientAuthenticationMode.Default;
                clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);
                
                // Load the page
                ClientSidePage page = clientContext.Web.LoadClientSidePage("IT.aspx");
                
                // Save the page as template for later use
                page.SaveAsTemplate("IT_Template.aspx");
            }
        }
    }
}