Currently the Youtube video Javascript prompt looks like:
<a href="JavaScript:paste_strinL(selektion,4,'[youtube='+prompt('YouTube%20movie%20URL:','http://youtu.be/')+']','','')" onmousemove="pasteSel()"><img src="https://www.minibb.com/forums/img/button_youtube.gif" style="width:23px;height:22px" alt="" title="Insert YouTube video"></a>
A small 'issue' is that after clicking on the Cancel button, the following is still pasted into the form:
[youtube=null]
It seems the code is almost the same as for the URL, so I'm not sure it behaves differently with the youtube link.. Ideally, there should be nothing pasted when user clicks on the Cancel button.