[Bill Posters]

e.g.

function doThis() {
if (document.forms.myform.mycheckbox.checked == true) {
document.forms.myform.mytextfield.value = document.forms.myform.myselection.value;
}
}

…

<select id="myselection" onchange="doThis()">
…
</select>

Change id and function names to suit.