我用mmm的提示解决了这个问题，并创建了一个简单的自定义REST端点。

WP1（前端，发送数据）：

function send_form_data_to_backend($form_values) {

    // add secret-key
    $form_values[\'secret_key\'] = "abcde123";

    $url = \'http://backend.example.com/wp-json/example/v1/save/\';
    $response = wp_safe_remote_post( $url, array(
        \'method\' => \'POST\',
        \'timeout\' => 15,
        \'redirection\' => 5,
        \'blocking\' => true,
        \'headers\' => array(),
        \'body\' => $form_values,
        \'cookies\' => array()
        )
    );

    if ( is_wp_error( $response ) ) {
       $error_message = $response->get_error_message();
       echo "Something went wrong: $error_message";
    } else {
       echo \'Response:<pre>\';
       print_r( $response );
       echo \'</pre>\';
    }
}

add_action( \'rest_api_init\', \'post_request\' );

function post_request() {

    register_rest_route( \'example/v1\', \'/save/\', array(
                \'methods\' => \'POST\',
                \'callback\' => \'rest_function\')
    );
}


/*
 * REST Function
 */
function rest_function( $data ) {

    if ( $data[\'secret_key\'] == "abcde123" ) {

        $msg = example_main_form_processing( $data );

        return $msg;

    } else {
        return "Wrong Key.";
    }

}