>John, you need to compose the JSON data and post it.
>LOCAL loXMLHTTP as "MSXML2.XMLHTTP"
>loXmlHttp = CREATEOBJECT("MSXML2.XMLHTTP")
>loXmlHttp.open("POST", cdom, .F., cUser, cPassw)

>TEXT TO JSONData NOSHOW
>{ "username": "hbelalcazar@xsoftware.com.co",
> "password": "abcfghz$",
> "client": "companyx"
>}
>ENDTEXT

>loXmlHttp.send(m.JSONData)
>Of course, not only this does not pretend to be real production code, but also you should use a JSON library (besides Rick's, you can also check Marco Plaza's nfJson at VFPX) to write and read the webapi data.

Thank you Antonio and also Mike and Rick, I received some additional help from the provider of the webapi and the final code that worked for me is this:

LOCAL loXMLHTTP as "MSXML2.XMLHTTP", lAsync as Boolean, JSONCred as String
loXmlHttp = CREATEOBJECT("MSXML2.XMLHTTP") 
lAsync=.T. &&asynchronohus
cUser="hbelalcazar@xsoftware.com.co"
cPass="abcfghz$"
cdom="https://xxxtapi.awswebsites.net/api/Internal_list"
JSONCredentials = '{ "username": "hbelalcazar@xsoftware.com.co", "password": abcfghz$", "client": "companyx" }'

loXmlHttp.open("POST", cdom, lAsync) && no need to send user and password here
loXmlHttp.setRequestHeader("Content-type:", "application/json")  && This was the last line added
loXmlHttp.setRequestHeader("Accept:", "application/json") 
loXmlHttp.send(JSONCredentials) &&returns the data OK!