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:
Some plain 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.