Add a link below every media in media gallery

functions.php

<?php

/**
* Add a link below every media in media gallery.
*/
function tmp_rtmedia_rtmedia_after_item_callback() {
	global $rtmedia_media;

	// Check if $rtmedia_media->id rtMedia id is set  and also check for if $rtmedia_media->media_id post id is set
	if ( isset( $rtmedia_media->id ) && isset( $rtmedia_media->media_id ) ) {

		// rtMedia id.
		$media_id = $rtmedia_media->id;

		// Post id.
		$post_id = $rtmedia_media->media_id;

	    ?>
	    <a href="http://demo.rtmedia.io/" class="no-popup media-id-<?php echo intval( $media_id ); ?> post-id-<?php echo intval( $post_id ); ?>">Buy Now</a>
	    <?php
	}
}
add_action( 'rtmedia_after_item', 'tmp_rtmedia_rtmedia_after_item_callback', 99 );