According to API Ninja's documentation (link), you need to set the X-Api-Key header.

Here is an untested example:
Code:
Public Async Function GetStockPrice(ticker As String) As Task(Of String)
    If (String.IsNullOrWhitespace(ticker)) Then
        Throw ArgumentOutOfRangeException(NameOf(ticker))
    End If

    Dim baseUrl As String = "https://api.api-ninjas.com/v1/stockprice"
    Dim query As NameValueCollection = HttpUtility.ParseQueryString($"ticker={ticker}")
    Dim url As String = $"{baseUrl}?{query.ToString()}"
    Dim apiKey As String = "CHANGE ME"

    Using client As New HttpClient()
        Using request As New HttpRequestMessage(HttpMethod.Get, url)
            request.Headers.Add("X-Api-Key", apiKey)

            Using response As HttpResponseMessage = Await client.SendAsync(request)
                response.EnsureSuccessStatusCode()

                Dim responseBody As String = Await response.Content.ReadAsStringAsync()
                Return responseBody
            End Using
        End Using
    End Using
End Function