If you don't need any formatting, you can add plain text into your notes in two ways:
 

1. Prefix any string that causes formatting with \ symbol.

\[youtube test]
 

2. Wrap a text into [text] and [/text] blocks

Like this:
[text]
Some plain text
[/text]
 
You can also omit the final [/text] block, this way the note will be treated as plain text from its [text] block and until its end.