Vendors that offer API's will usually offer code snippets in various languages. If you choose an XML over http interface you will need to make a reference to a version of Microsoft XML(XX), not positive what the reference name looks like but there are typically only a couple options available. I usually grab the latest version.
I took a quick look at clickatell's website and did not see any sample snippets. Here is one for a basic text message. I cannot remember if it is the SMS protocol but I believe it is. I got this from a text file within one of my project's folders (as a snippet). The string that you POST and the HEADER will be specific to your vendor's API.
Code:
If IsNothing(Me.txtMsg.Value) Then 'Custom UDF
MsgBox "Please type your message in the space provided", vbInformation, "Need Msg"
Exit Sub
End If
If IsNothing(Me.txtNumber.Value) Then 'Custom UDF
MsgBox "Please enter the phone number or select one from the list.", vbInformation, "No Number"
Exit Sub
End If
Dim strMsg As String
Dim strNumber As String
strMsg = Me.txtMsg.Value
strNumber = Me.txtNumber.Value
Dim sUrl
Dim sAPI_ID, sPassword, sUsername, sMobileNo, sText
Dim oXMLHTTP As Object
Dim sPostData, sResult
sUrl = "https://some.address.here"
sAPI_ID = "XXX"
sPassword = "XXX"
sUsername = "XXX"
sMobileNo = "1" & strNumber 'International code and number
sText = strMsg
sPostData = "api_id=" & sAPI_ID
sPostData = sPostData & "&user=" & sUsername
sPostData = sPostData & "&password=" & sPassword
sPostData = sPostData & "&to=" & sMobileNo
sPostData = sPostData & "&text=" & sText
Set oXMLHTTP = CreateObject("Microsoft.XMLHTTP")
oXMLHTTP.Open "POST", sUrl, False
oXMLHTTP.SetRequestHeader "Content-Type", "application/x-www-form-urlencoded"
oXMLHTTP.Send sPostData
sResult = oXMLHTTP.ResponseText
Set oXMLHTTP = Nothing
'Response.Write sResult
If fWrite_Text_Confirmation(sResult) Then 'Custom UDF
MsgBox "Your message has been sent."
End If