calling extension methods and calling them in data contracts

Hi,
I have using a 3 party payment cart provided by chase.
below is a extension method. Irequest is the object provided by chase for sending them a request.
String ordernumber is anything string.

Suppose , i want to call this SetMerchantOrderNumber(this IRequest request, string orderNumber)  in another class, how do i call it. i know i have to using this class using using in class file.

also, how do i mport this into datacontracts for sending the request to chase using wcf.
using System;
using System.Collections.Generic;
using System.Linq;
using ChasePaymentech.Spectrum;
using Vertrue.Core.String;
using Vertrue.Core.PaymenTech.Exceptions;
using Vertrue.Core.PaymenTech.Extensions.CreditCardVendors;
using Vertrue.Core.PaymenTech.Interfaces;

namespace Vertrue.Core.PaymenTech.Extensions.Request
{
    public static class IRequestExtensions
    {
        /// <summary>
        /// Validates the value passed in does not contain invalid characters and sets the MerchantOrderNumber property on the request.
        /// </summary>
        /// <param name="request">Any Paymentech request.</param>
        /// <param name="orderNumber">A string value must not contaion |,^,%,\,/,[,],`,~ characters and must be less than 23 characters.</param>
        public static void SetMerchantOrderNumber(this IRequest request, string orderNumber)
        {
            if (!string.IsNullOrWhiteSpace(orderNumber))
            {
                var invalidCharacters = new HashSet<char> { '|', '^', '%', '\\', '/', '[', ']', '~', '`' };
                const int merchantOrderNumbermaxLength = 22;

                //check for invalid characters
                if (orderNumber.ToCharArray().Any(invalidCharacters.Contains))
                    throw new ArgumentContainsInvalidCharacterException("value", invalidCharacters);

                //check for invalid length
                if (!orderNumber.IsValidLength(merchantOrderNumbermaxLength))
                    throw new ArgumentToLongException("value", merchantOrderNumbermaxLength);

                //should be left aligned and padded with spaces.
                orderNumber = orderNumber.Trim().Length == 0 ? null : orderNumber.PadRight(22, ' ');
            }
            else
                orderNumber = null;

            const string merchantOrderNumberKey = "MerchantOrderNumber";
            request[merchantOrderNumberKey] = orderNumber;
        }