Determination of URL to direct HTTP POST's to using Microsoft.XMLHTTP object?

[MLH]

http://www.alltel.com/text_messaging/
Above is the URL to which one would direct his browser to send text
message to Alltel customer. I wrote a VBA script to automate text
messaging field personnel with a construction company last year. All
field reps carried Arch pagers. Arch.com is NOT affiliated with
Alltel. Faced with a similar objective, how do I go about determining
the URL I need to use with Alltel? I don't imagine its the same as the
public site above.

I'm uncertain what URL to make an HTTP POST to in order to automate
the sending of text messages to new customer sales force. All carry
Alltel phones with text-mssaging capability. Am uncertain how rkc
determined the URL. Anyone know???

rkc gave me some pointers on Sun, Oct 9 2005 7:08 pm regarding
Get having been disabled on Arch servers.

Dim msXML As Object, strPageContent As String, MyURL As String
Set msXML = CreateObject("Microsoft.XMLHTTP")
MyURL = "http://www.Arch.com/cgi-bin/wwwpage.exe"
msXML.Open "POST", MyURL, False
<snip>
msXML.Send _
("PIN=8005551234&MSSG=This+is+the+text+of+the+mess age&Q1=0")
strPageContent = msXML.responseText