尝试在正文中以 body 或 json 形式发送用户名和密码，而不是 >application/x-www-form-urlencoded，像这样:

Try sending username and password inside of a body as body or json, instead of as application/x-www-form-urlencoded, like so:

$res = $client->post('http://stage.example.co.uk/partnerhubwebservice.asmx/Authorise', [
    'headers' => [
        'Content-Type' => 'application/x-www-form-urlencoded',
    ],
    'json' => [
        'username' => 'MMM_bookings',
        'password' => 'passwordaaaa',
    ]
]);

或

$res = $client->post('http://stage.example.co.uk/partnerhubwebservice.asmx/Authorise', [
    'headers' => [
        'Content-Type' => 'application/x-www-form-urlencoded',
    ],
    'body' => [
        'username' => 'MMM_bookings',
        'password' => 'passwordaaaa',
    ]
]);