How to send a form-data POST request with VBA in Excel using WinHTTPRequest

I am building an API using FastAPI that must be accessible from Excel VBA. FastAPI's OAuth2 authentication mechanism requires me to send a "form-data" POST request, but I don't know how to do this using WinHTTPRequest in VBA.

I already have a function which get plain JSON from the API, but this is a GET function and I'm not sure where to specify the "form-data" part in the body, nor where to put the username and password key-value pairs.

Here is a simple VBA GET that handles some errors. How would I modify this to do a form-data POST with username and password fields?

Public Function getreq(url As String)    Dim req As WinHttpRequest    Dim JsonString As String    Dim jp As Object    Dim resp As String    Dim errorstring As String    Set req = New WinHttpRequest' req.SetRequestHeaderxxx ?' this is where auth will go via POST form-data username and password?    req.Open "GET", url    On Error GoTo errhand:        req.Send        resp = req.ResponseText        If resp = "Internal Server Error" Then            resp = "{'error': 'Internal server error'}"        End If        getreq = resp    Exit Functionerrhand:    Select Case Err.Number        Case -2147012894 'Code for Timeout            getreq = "{'error': 'Request timeout'}"        Case -2147012891 'Code for Invalid URL            getreq = "{'error': 'Bad url'}"        Case -2147012867 'Code for Invalid URL            getreq = "{'error': 'Cannot establish connection'}"        Case Else 'Add more Errorcodes here if wanted            errorstring = "Errornumber: "& Err.Number & vbNewLine & "Errordescription: "& Error(Err.Number)            getreq = "{'error': '"& errorstring & "'}"    End SelectEnd Function