How do i send an api key in a request header

I'm using a service that provides an api to get data out of our online, hosted database. I've never called data this way from VBA and am wondering how to do it.
I have an api key, username, password but am not sure how to put code together to make it work.
Code I've put together is below:

Dim httpreq As New MSXML2.XMLHTTP60, myurl As String
myurl = "https://cowtown.breezechms.com/api/people"
httpreq.Open "GET", myurl, False
httpreq.setRequestHeader "Content-Type", "application/json"
httpreq.setRequestHeader "dduck", "abc123"
httpreq.send
MsgBox (httpreq.responseText)

The MsgBox just kicks back the following response.

"Permission Denied - API key () does not match subdomain (cowtown)"