mrp_structured_data_type_product_main_entity_example.php

<?php
/**
 * Adds example mainEntityofPage to Product type based on WordPress SEO graph
 */
function mrp_structured_data_type_product_main_entity_example( $schema, $post_id ) {

	$structured_data_type = get_post_meta( $post_id, MRP_Multi_Rating::STRUCTURED_DATA_TYPE_POST_META, true );

	if ( $structured_data_type === 'Product' ) {
		return '
"mainEntityOfPage": "' . get_the_permalink( $post_id ) . '#webpage",
';
	}    
    return '';
}
add_filter( 'mrp_structured_data_type' , 'mrp_structured_data_type_product_main_entity_example', 10, 2);