How to force/Manually add authenticaion to POST WebRequest

I need to provide authentication for a POST request. I have never done this before so I have been searching for methods to accomplish this.
I have found this method but it did not work. The message I received was 401 Unauthorized.

Dim myCred As New NetworkCredential("USERNAME", "PASSWORD")
        Dim credsCache As New CredentialCache()
        credsCache.Add(New Uri(Uri), "Basic", myCred)
        Dim request As WebRequest = WebRequest.Create(Uri)
        request.Credentials = credsCache

I found other posts indicating that sometimes the authentication has to be forced. I have been trying to get this code to work but I do not know how to declare the AUTHORIZATION

Dim request As WebRequest
Dim credidentials As String = "userName" + ":" + "Password"
Authorization = Convert.ToBase64String(Encoding.[Default].GetBytes(credidentials))  'How do I declare Authorization?
request.Headers("Authorization") = "Basic " + Authorization

How do I declare authorization or how do I "force" authorization?

Thanks,
pat