Change the script of the send sms page as below;
OLD: $objIntelliSMS->Username = 'username';
UPDATE WITH: $objIntelliSMS->Username = $_POST['username'];
OLD: $objIntelliSMS->Password = 'password';
UPDATE WITH: $objIntelliSMS->Password = $_POST['password'];
And input two input fields in the form labelled 'username' and password' and the user can input the username and password and these input details will also be forwarded to intellisoftware with the other input data.