custom-field-insert.js

<script>

Event.observe( document, 'dom:loaded', beg_el = function() {
var textValue = $('ticket_fields_139051').value;
return textValue;
},
beg_nl = function() {
var textValue = $('ticket_fields_114704').value;
return textValue;});


</script>

<div style="text-align: center; padding:5px;">
<p style="font-weight: bold;font-color: #1a1a1a;margin:0 0 10px 7px;text-align:left;">opening</p>
<input type="button" id="beg_nl" value="username" onClick="$('comment_value').value += beg_nl();" />
<input type="button" id="beg_en" value="password" onClick="$('comment_value').value += beg_el();" />
</div>

inserting custom field data into a ticket comment