Hi,

Please install the Code Snippet plugin, then add the following code:

function waerp_filter_post_json( $data, $post, $context ) {
    $postmetas = get_post_meta($post->ID);
    foreach($postmetas as $meta_key=>$meta_value) {
        $data->data[$meta_key] = $meta_value[0];
    }
    return $data;
}
add_filter( 'rest_prepare_post', 'waerp_filter_post_json', 10, 3 );

This code will return all custom meta keys of post type name: post in REST API result.

Best Regards,
PenciDesign.