You will find the tag in the post toolbar.
Here's an example post with YouTube video embedded.
viewtopic.php?f=58&t=22123&p=57007#p57007
And here's the phpbb code
Code: Select all
[youtube]t1aNJVhUaYU[/youtube]
Code: Select all
[youtube]t1aNJVhUaYU[/youtube]