I have a asp.net web form i want to post data of my form in to marketo

Yes, here is the sample code from developers.marketo.com  http://developers.marketo.com/documentation/rest/createupdate-leads/

Sample Code - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Net;
using System.IO;
using System.Collections;
using Newtonsoft.Json;
namespace Samples
{
    class UpsertLeads
    {
        private String host = "CHANGE ME"; //host of your marketo instance, https://AAA-BBB-CCC.mktorest.com
        private String clientId = "CHANGE ME"; //clientId from admin > Launchpoint
        private String clientSecret = "CHANGE ME"; //clientSecret from admin > Launchpoint
        public String lookupField;//field to deduplicate on
        public String action;//action to take, createOnly, updateOnly, createOrUpdate, createDuplicate
        public List<Dictionary<string, string>> input;//list of dictionaries which represent field/value pairs of leads
        public String partitionName;
        /*
        public static void Main(String[] args)
        {
            UpsertLeads upsert = new UpsertLeads();

This method is significantly simpler and easier to implement than Jamie's suggestion: Make a Marketo Form Submission in the background