Imports System.Web.Script.Serialization
Imports Newtonsoft.Json

Public Class Form1
    Private Sub go_button_Click(sender As Object, e As EventArgs) Handles go_button.Click

        Dim json As String = input_box.Text
        Dim posts = JsonConvert.DeserializeObject(Of Post())(json) ' Deserialize array of Post objects

        If posts.Length = 1 Then ' or whatever condition you prefer
            post_text_box.Text = posts(1).com
        End If


    End Sub
End Class

Public Class Rootobject
    Public Property posts() As Post
End Class

Public Class Post
    Public Property no As Integer
    Public Property now As String
    Public Property name As String
    Public Property com As String
    Public Property filename As String
    Public Property ext As String
    Public Property w As Integer
    Public Property h As Integer
    Public Property tn_w As Integer
    Public Property tn_h As Integer
    Public Property tim As Long
    Public Property time As Integer
    Public Property md5 As String
    Public Property fsize As Integer
    Public Property resto As Integer
    Public Property bumplimit As Integer
    Public Property imagelimit As Integer
    Public Property replies As Integer
    Public Property images As Integer
End Class