I finally managed to solve it and it turned out to be a pretty easy fix (I'm a bit surprised noone here could help me in the right direction...)
I needed to use a POST rather than a GET....
Here's the working code in case someone ever needs it.
Dim pword$
Sub cchk()
Dim uname$, curl$ oXml As Object
uname = Environ("USERNAME")
curl = "http://www.checkprice.nl/index.php?a=product_specs&product_id=" & Target.Value
If pword = "" Then pword = InputBox("Password")
dts = "username=" & uname & "&password=" & pword
Set oXml = CreateObject("MSXML2.XMLHTTP")
oXml.Open "POST", curl, False
oXml.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
oXml.send dts
Range("C" & Target.Row).Value = oXml.responsetext
Set oXml = Nothing
End Sub
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