This will be because laravel is expecting a csrf token in a post and it won’t be present in the request from your externals url's post request.
To get round this you can exclude the remote url from being checked for a token.
namespace App\Http\Middleware;
use Illuminate\Foundation\Http\Middleware\VerifyCsrfToken as Middleware;
class VerifyCsrfToken extends Middleware
{
/**
* The URIs that should be excluded from CSRF verification.
*
* @var array
*/
protected $except = [
'stripe/*',
'http://example.com/foo/bar',
'http://example.com/foo/*',
];
}