How to use wpseo_pre_analysis_post_content in WP SEO by Yoast?

howto_preanalysis_post_content.php

<?php
/**
 * Add (custom field) content to the post's content before WP SEO Analysis.
 *
 * @param string $content The content of a posts content-field. 
 *
 * @return string The original content plus the value of the postmeta with meta_key '_yoast_postmeta_example'.
 */
function yst_custom_content_analysis( $content ) {
	global $post;

	return $content . ' ' . get_post_meta( $post->ID, '_yoast_postmeta_example', true );
}

add_filter( 'wpseo_pre_analysis_post_content', 'yst_custom_content_analysis' );

Hello, Can you please let me know where to add this script?

Add it to your functions.php file in your theme

Just to say, code like this will no longer work as this hook has been removed by Yoast due to it requiring AJAX and slowing down the plugin. You need to use a js plugin as detailed here.