Source

File: wp-includes/link-template.php

function get_preview_post_link( $post = null, $query_args = array(), $preview_link = '' ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return;
	}

	$post_type_object = get_post_type_object( $post->post_type );
	if ( is_post_type_viewable( $post_type_object ) ) {
		if ( ! $preview_link ) {
			$preview_link = set_url_scheme( get_permalink( $post ) );
		}

		$query_args['preview'] = 'true';
		$preview_link          = add_query_arg( $query_args, $preview_link );
	}

	/**
	 * Filters the URL used for a post preview.
	 *
	 * @since 2.0.5
	 * @since 4.0.0 Added the `$post` parameter.
	 *
	 * @param string  $preview_link URL used for the post preview.
	 * @param WP_Post $post         Post object.
	 */
	return apply_filters( 'preview_post_link', $preview_link, $post );
}