[Warlord]

To install this on the quick reply edit the showthread_quickreply template

Find:

PHP Code:

        <td><div class="imagebutton" id="{$editorid}_cmd_wrap0_quote"><img src="$stylevar[imgdir_editor]/quote.gif" width="21" height="20" alt="$vbphrase[wrap_quote_tags]" /></div></td>
    </if> 


Add after:

PHP Code:

<td><div class="imagebutton" id="{$editorid}_cmd_wrap0_spoiler"><img src="$stylevar[imgdir_editor]/spoiler.gif" alt="[SPOILER]" width="21" height="20" border="0" /></div></td> 


EDIT: I created my own BB Code the old fashioned way and did not install this mod so to speak, but I don't see why it wouldn't work on 3.8

If not, just add the following BB Code.

In the Admin CP Click Custom BB Codes then select Add New BB Code. Fill in the blanks with this information.

Title: Spoiler
BB Code Tag Name: spoiler
Replacement:

<div style="margin:5px 20px 20px 20px">

<div class="smallfont" style="margin-bottom:2px"><b>Spoiler:</b> 

<input type="button" value="Click to Reveal" style="width:100px;font-size:10px;margin:0px;padding:0px;" onclick="if (this.parentNode.parentNode.getElementsByTagName('div')[1].getElementsByTagName('div')[0].style.display != '') { this.parentNode.parentNode.getElementsByTagName('div')[1].getElementsByTagName('div')[0].style.display = '';this.innerText = ''; this.value = 'Hide'; } else { this.parentNode.parentNode.getElementsByTagName('div')[1].getElementsByTagName('div')[0].style.display = 'none'; this.innerText = ''; this.value = 'Show'; }">

</div>

<div class="alt2" style="margin: 0px; padding: 6px; border: 1px inset;">

<div style="display: none;">

{param}

</div>

</div>

Example: [spoiler]This text is Animation Spoiler[/spoiler]
Description: The [spoiler] tag allows you hide your text.

Make sure you upload the attached image to forumroot/images/editor.