I mean this is an extremely simple PHP page. Literally just very basic CSS to make the whole page black and then YouTube provides their embed code which you can just copy paste and grab the video ID from the URL. If you're having to ask how to do this then likely you just need to learn some basic PHP because this is essentially a result you could get easily from any of the thousands of introductory PHP tutorials out there.
If you're still having trouble I'd suggest posting in the Plugin Development section because you'll get better coding help in that forum. Though again, at least Google creating a simple PHP page before you post in there, this is very simple stuff you're requesting.
Here
1. Make a new file called
video.php with this content:
<?php
define( 'IN_MYBB', 1 );
require_once './global.php';
if( empty( $mybb->input['v'] ) )
{
error( 'Specify a video id.' );
}
else
{
$v = $mybb->input['v'];
eval( '$page = "' . $templates->get( 'video' ) . '";' );
output_page( $page );
}
2. Make a new template called
video with this content
<style>
body {
background: #000
}
.container {
position: relative;
width: 100%;
height: 0;
padding-bottom: 56.25%;
}
.video {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
</style>
<div class="container">
<iframe src="//www.youtube.com/embed/{$v}?autoplay=1" frameborder="0" allowfullscreen class="video"></iframe>
</div>
Result:
https://a.pomf.cat/pudqeb.mp4