Hmm, we use a very custom style that has almost nothing in common with the default style ^.^
Lemme check ...
In Template pm_newpm FIND and REMOVE
else if (formname.dopreview != true)
{
<if condition="$show['trackpm']">
if (confirm("$vbphrase[request_receipt_for_message]"))
{
formname.receipt.value = 1;
}
</if>
}
FIND and REMOVE
<input type="hidden" name="receipt" value="0" />
FIND
<div><label for="cb_parseurl"><input type="checkbox" name="parseurl" tabindex="1" $checked[parseurl] />$vbphrase[automatically_parse_links_in_text]</label></div>
BELOW that ADD
<if condition="$show[trackpm]">
<div><label for="cb_receipt"><input type="checkbox" name="receipt" value="1" id="cb_receipt" tabindex="1" <if condition="$bbuserinfo[fieldx]">checked="checked"</if> />$vbphrase[request_receipt_for_message]</div>
</if>
Where fieldx is the custom profile field for setting the default.