Passing API key and parameters using VBA in a POST request - json

Question

I have been trying to wrap my head around this VBA-Web doc http://vba-tools.github.io/VBA-Web/docs/ in order to pass auth_key and other parameters in a post request. I understand the concept of encoding the auth_key as explained here How to pass API key in VBA for Get and Post Request?. But I need help to include all the parameters below in VBA.
UPDATE:
I was able to make QHarr's code below work. But I'm still finding it hard to use my own curl parameters below. I've tried using EncodeBase64(apikey) but I'm not there yet.
Public Sub GetResults()
Dim data As String, json As Object '< VBE > Tools > References > Microsoft Scripting Runtime
data = "{""domainNames"":[""google.com""]}"
With CreateObject("WinHttp.WinHttpRequest.5.1")
.Open "POST", "https://api.dev.name.com/v4/domains:checkAvailability", False
.setRequestHeader "User-Agent", "Mozilla/5.0"
.setRequestHeader "Content-Type", "application/x-www-form-urlencoded; charset=UTF-8"
.setRequestHeader "Authorization", "Basic " + _
EncodeBase64("username" + ":" + "Token")
.send data
Set json = JsonConverter.ParseJson(.responseText)
Dim result As Object
For Each result In json("results")
Debug.Print result("domainName")
Next
End With
End Sub
Function EncodeBase64(text As String) As String
Dim arrData() As Byte
arrData = StrConv(text, vbFromUnicode)
Dim objXML As MSXML2.DOMDocument60
Dim objNode As MSXML2.IXMLDOMElement
Set objXML = New MSXML2.DOMDocument60
Set objNode = objXML.createElement("b64")
objNode.DataType = "bin.base64"
objNode.nodeTypedValue = arrData
EncodeBase64 = Application.Clean(objNode.text)
Set objNode = Nothing
Set objXML = Nothing
End Function
Here is the API doc - https://cloud.ibm.com/apidocs/
Feature request:
curl -X POST \
-H "Content-Type: application/json" \
-u "apikey:{apikey}" \
-d #parameters.json \
"{url}/v1/analyze?version=2019-07-12"
Example parameter:
{
"features": {
"semantic_roles": {}
},
"text": "IBM has one of the largest workforces in the world"
}