i am trying to use rtmp://

WordPress doesn’t have the rtmp protocol whitelisted. There is, however, a filter that lets you add it. This plugin should do the trick:

<?php
/*
Plugin Name: Allow rtmp:// links
 */

function allow_rtmp_links( $protocols ) {
	$protocols[] = 'rtmp';
	return $protocols;
}

add_filter( 'kses_allowed_protocols', 'allow_rtmp_links' );