There is this youtube plugin that may work,
https://www.dokuwiki.org/plugin:youtube
If it doesn't do what you want, you can still use
<iframe>, but you must first allow HTML in doku configuration. You should allow HTML
only if all users on your wiki are trusted.
Note that you have to surround the
<iframe>...</iframe>[/m] with [m]<HTML>[/m] [m]</HTML> doku markup.