Add a string of text into an input field when user clicks a button

this will do it with just javascript - you can also put the function in a .js file and call it with onclick

//button
<div onclick="
   document.forms['name_of_the_form']['name_of_the_input'].value += 'text you want to add to it'"
>button</div>

Example for you to work from

HTML:

<input type="text" value="This is some text" id="text" style="width: 150px;" />
<br />
<input type="button" value="Click Me" id="button" />​

jQuery:

<script type="text/javascript">
$(function () {
    $('#button').on('click', function () {
        var text = $('#text');
        text.val(text.val() + ' after clicking');    
    });
});
<script>

Javascript

<script type="text/javascript">
document.getElementById("button").addEventListener('click', function () {
    var text = document.getElementById('text');
    text.value += ' after clicking';
});
</script>

Working jQuery example: http://jsfiddle.net/geMtZ/ ​