<form method="post" action="object-transfers.php">
<input type="hidden" value="39975" name="id"/>
<input type="hidden" value="uran" name="resource"/>
<input type="hidden" value="1257932450" name="sign_value"/>
<input type="hidden" value="148e33492490ce4a12dd338e0aa3282b" name="sign_md5"/>
<input type="hidden" value="99" name="checkprice"/>
<input type="hidden" value="sell" name="action"/>
<nobr>
<input type="text" size="5" value="0" name="amount"/>
<input type="submit" style="background: rgb(224, 238, 224) none repeat scroll 0% 0%; -moz-background-clip: border; -moz-background-origin: padding; -moz-background-inline-policy: continuous;" value=">>"/>
</nobr>
</form>

var elem = document.getElementById('elem_id');
elem.setAttribute('имя_атрибута', 'значение');

<script>
function ymnojaem(x, y){
return x * y
}
</script>
<textarea id = "x" cols=1 rows=2> 
<textarea id = "y" cols=1 rows=2>
<textarea id = "otvet" cols=1 rows=5>
<button onclick="document.all.otvet. value=ymnojaem(x, y)">умножить</button>

<script>
function ymnojaem(x, y){
    return x * y
}
</script>
<textarea id = "x" cols=1 rows=2></textarea>
<textarea id = "y" cols=1 rows=2></textarea>
<textarea id = "otvet" cols=1 rows=5></textarea>
<button onclick="document.getElementById('otvet').value = ymnojaem(document.getElementById('x').value, document.getElementById('y').value)">умножить</button>