How to Emulate a POST Request from PostMan - For a series of Integration Tests in a WebAPI Development - vb.net

Question

I am using PostMan as part of the testing regime for a WebAPI service development that I am working on. In order for external parties to gain access to our WebAPI service they first need to obtain an access token.
The POST request returns some JSON containing the required access token:
{
"access_token": "anencryptedaccesstoken",
"scope": "am_application_scope default",
"token_type": "Bearer",
"expires_in": 3218
}
I am putting together a series of integration tests which need to emulate the POST calls from POSTMAN. I am currently using System.Net.WebClient to achieve this. I am not sure what I need to do in order to achieve my goal. Here is a function that I am using to try and obtain the access token:
Public Shared Function GetAccessToken(ByVal endpoint As String, wc As WebClient) As String
Dim result As String = ""
Dim data As Byte() = Nothing
'Header information
wc.Headers.Add("Authorization", "Basic <alongencryptedstring>")
wc.Headers.Add("Content_Type", "application/x-www-form-urlencoded")
result = wc.UploadString(endpoint, "POST", "")
Return result
End Function
The 'Body' tab in PostMan contains the following entries:
grant_type - client_credentials
Content_Type - application%2Fx-www-form-urlencoded
In this instance as far as I am aware there is no 'data' element to the PostMan request hence the empty string in my use of UploadString. The function returns the following error:
"The remote server returned an error: (415) Unsupported Media Type."
I am new to web app development so please bear with me, I am finding it difficult to phrase what I think are meaningful question in the context of this post, I hope however that I have ssupplied sufficient information to convey the gist of my problem and for someone to be able to respond appropriately.
Kind Regards
Paul.