Hi @gfy,

Thank you for using wpForo and for contacting us. 

Please put this code in the active WordPress theme's functions.php:

add_filter('wpforo_content_after', 'wpforo_custom_video_embed', 10);
function wpforo_custom_video_embed( $content ){
    $paterns = array();
    $paterns[] = "/<a[^><]+>\s*[a-zA-Z\/\/:\.]*youtube.com\/watch\?v=([a-zA-Z0-9\-_]+)([a-zA-Z0-9\/\*\-\_\?\&\;\%\=\.]*)/i";
    $paterns[] = "/<a[^><]+>\s*[a-zA-Z\/\/:\.]*youtu.be\/([a-zA-Z0-9\-_]+)([a-zA-Z0-9\/\*\-\_\?\&\;\%\=\.]*)/i";
    $content = preg_replace($paterns, "<iframe width=\"420\" height=\"280\" src=\"//www.youtube.com/embed/$1\" frameborder=\"0\" allowfullscreen></iframe>", $content);
    return $content;
}

For more embedding options I recommend wpForo Embeds addon:

https://gvectors.com/product/wpforo-embeds/

Thanks!