使用VB.Net中的HttpClient进行POST请求并发送JSON字符串，可以按照以下步骤进行操作：

1. 首先，将以下命名空间添加到您的VB.Net项目中：

Imports System.Net.Http
Imports System.Text

2. 创建一个HttpClient实例，并设置基本的请求头信息：

Dim client As New HttpClient()
client.DefaultRequestHeaders.Accept.Clear()
client.DefaultRequestHeaders.Accept.Add(New MediaTypeWithQualityHeaderValue("application/json"))

3. 创建一个用于发送POST请求的方法，其中包括要发送的JSON字符串和目标URL：

Private Async Function PostJsonAsync(ByVal json As String, ByVal url As String) As Task(Of HttpResponseMessage)
    Dim content As New StringContent(json, Encoding.UTF8, "application/json")
    Dim response As HttpResponseMessage = Await client.PostAsync(url, content)
    Return response
End Function

4. 调用上述方法，并传入要发送的JSON字符串和目标URL：

Dim json As String = "{""key"": ""value""}" ' 替换为您要发送的JSON字符串
Dim url As String = "http://example.com/api" ' 替换为目标URL
Dim response As HttpResponseMessage = Await PostJsonAsync(json, url)

5. 处理响应，例如获取响应内容：

Dim responseContent As String = Await response.Content.ReadAsStringAsync()
Console.WriteLine(responseContent)

完整的示例代码如下：

Imports System.Net.Http
Imports System.Text

Module Module1

    Sub Main()
        Dim json As String = "{""key"": ""value""}" ' 替换为您要发送的JSON字符串
        Dim url As String = "http://example.com/api" ' 替换为目标URL

        Dim response As HttpResponseMessage = PostJsonAsync(json, url).GetAwaiter().GetResult()

        Dim responseContent As String = response.Content.ReadAsStringAsync().GetAwaiter().GetResult()
        Console.WriteLine(responseContent)
    End Sub

    Private Async Function PostJsonAsync(ByVal json As String, ByVal url As String) As Task(Of HttpResponseMessage)
        Dim client As New HttpClient()
        client.DefaultRequestHeaders.Accept.Clear()
        client.DefaultRequestHeaders.Accept.Add(New MediaTypeWithQualityHeaderValue("application/json"))

        Dim content As New StringContent(json, Encoding.UTF8, "application/json")
        Dim response As HttpResponseMessage = Await client.PostAsync(url, content)
        Return response
    End Function

End Module

请注意，上述示例中的URL和JSON字符串仅供示范目的，您需要根据自己的实际需求进行替换。另外，此示例使用了异步编程模型，因此需要使用Async和Await来处理异步操作。