If the Plain text editor is used a simple copy and paste of the (youtube) embedding html source code will be enough. The p.asp will take care of it as it has been programmed to recognize such code and will try to use it properly..
But if we use the fck editor, which is really like a word processor, pasting the same code will not work because the fck editor will take it as a simple text stream. The way to make the fck editor understand that you are pasting html code is to press the Source button . It will toggle the editor back and forth from the wysiwyg (what you see is what you get) display to the source code display. While in the source display the code can be pasted. Just don't paste inside an existing code stream.
Pressing the Source button again will put you back to the wysiwyg display.