<script language="JavaScript">
function test(element_name)
{
    document.foo.element_name.value = 'Hello';
}
</script>
...
<form name="foo">
<input type="text" name="text_el">
<button type="button"  onClick="test('text_el')">...</button><br>
<textarea name="textarea_el"></textarea>
<button type="button"  onClick="test('textarea_el')">...</button><br>
</form>

<script language="JavaScript">
function test(obj)
{
    obj.value = 'Hello';
}
</script>
...
<form name="foo">
<input type="text" name="text_el">
<button type="button"  onClick="test(document.foo.text_el)">...</button><br>
<textarea name="textarea_el"></textarea>
<button type="button"  onClick="test(document.foo.textarea_el)">...</button><br>
</form>

...
<tr id="m_select_fields" >
  <td class="cell" >Список полей SELECT-запроса *</td>
  <td class="cell"><textarea id="m_select_fields" class="input" name="m_select_fields">*</textarea>
   <button type="button" name="btn_m_select_fields" 
onclick="callTableFieldsDialog('m_select_fields');" ><b>...</b></button>
...

<button type="button" name="btn_m_select_fields" 
onclick="document.form_name.m_select_fields.value=showModalDialog(...);" ><b>...</b></button>

<button onClick="document.form_name.m_select_fields.value=callTableFieldsDialog();">...</button>