Idea presa su stackoverflow.
codice HTML:
<!DOCTYPE html>
<html>    
<head>    
</head>
    <body>
        <script>
            function showUser(val) {
                document.getElementById('a').innerHTML = val;
            }

            function pippo(val) {
                var a    = document.getElementById("nome");
                a.value += val;
                var evnt = a["onkeyup"];
                if (typeof(evnt) == "function") {
                    evnt.call(a);
                }
            }
        </script>
Msg <input type="text" id="nome" onKeyUp="showUser(this.value)" size="30"/>
<div id="a"></div>
<hr/>Tastiera<hr/>
<Input type='button' value='A' onclick="pippo(this.value)" />
<Input type='button' value='B' onclick="pippo(this.value)" />
<Input type='button' value='C' onclick="pippo(this.value)" />
    </body>
</html>