I think RestSharp will be your best bet. It will work for C# or vb.net. I am fairly new to the use of RestSharp but it has worked well for me. You might have to tweak the code below a little bit since I can't test this answer without an api key but it should get you started. You will also need to install RestSharp via Nuget first and then import it into your class.
Dim key As String = 'your api key'
Dim client As New RestClient("https://some.thing.uk/fred/prices")
Dim pagesrequest = New RestRequest("\?productcode=ZZ99ABC" & "&x-api-key:" & key, Method.GET)
Dim response As IRestResponse = client.Execute(pagesrequest)
Dim textresponse As String = response.Content
'Display the response so you can check it.
textbox1.text=textresponse
Also, "\?productcode=ZZ99ABC" doesn't look right to me. You might want to try it withoug the "\".