ASP.net Identity Webservice Authentication - asp.net

Question

I've a simple ASP.net Webforms application with authentication using Identity+Owin. Based on this tutorial here:
https://learn.microsoft.com/en-us/aspnet/identity/overview/getting-started/adding-aspnet-identity-to-an-empty-or-existing-web-forms-project
I've also added a webservice as code given bellow:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using Newtonsoft.Json;
namespace webservice_1
{
/// <summary>
/// Summary description for WebService1
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
// [System.Web.Script.Services.ScriptService]
public class WebService1 : System.Web.Services.WebService
{
[WebMethod]
public DataSet getRequests()
{
DataSet ds = new DataSet();
string cs = ConfigurationManager.ConnectionStrings["IBS_5"].ConnectionString;
using (SqlConnection con = new SqlConnection(cs))
{
SqlCommand sqlComm = new SqlCommand("spSelRequests", con);
sqlComm.CommandType = CommandType.StoredProcedure;
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = sqlComm;
da.Fill(ds);
return ds;
}
}
[WebMethod]
public string getRqsts()
{
DataSet ds = new DataSet();
string cs = ConfigurationManager.ConnectionStrings["IBS_5"].ConnectionString;
using (SqlConnection con = new SqlConnection(cs))
{
SqlCommand sqlComm = new SqlCommand("spSelRequests", con);
sqlComm.CommandType = CommandType.StoredProcedure;
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = sqlComm;
da.Fill(ds);
return JsonConvert.SerializeObject(ds, Newtonsoft.Json.Formatting.Indented);
}
}
}
}
Everything is working fine. I just want to add authentication to the webservice. Currently webservice can be called directly and no security is there.
Authentication is working fine for web pages.
The webservices will be used for android/IOS applications.
I m clueless how should I provide authentication here. Also how should I manage cookies and sessions for webservice.
Note: MVC is not understood by me...so pls consider.