CakePHP 3.x - save not work in associate table

Need Help
1

I have below table structure.

Mandator table

class MandatorTable extends Table
{
public function initialize(array $config)
    {
        $this->table('mandators');

        $this->belongsToMany('Seminar', [
            'foreignKey' => 'mandator_id',
            'targetForeignKey' => 'seminar_id',
            'joinTable' => 'mandators_seminars'
        ]);
	}
}

Semiar table

class SeminarTable extends Table
{
    public function initialize(array $config)
    {
        $this->table('seminars');

        $this->belongsToMany('Mandator', [
            'foreignKey' => 'seminar_id',
            'targetForeignKey' => 'mandator_id',
            'joinTable' => 'mandators_seminars'
        ]);	
    }	
}

both table are belong to ‘mandators_seminars’ table

mandator_id
seminar_id

When I save data it’s save in seminar table but not in ‘mandators_seminars’ table

Query

$seminartable = $this->Seminar->newEntity();

$this->request->data['mandator'][0] = 1;

$seminardata = $this->Seminar->patchEntity($seminartable, $this->request->data);
$this->Seminar->save($seminardata)

request data

Array
(
    [bookable] => test
    [released] => aaa
    [linkable] => bb
    [name] => ccc
    [internalnote] => ddd
    [abstract] => ttt
    [description] => ddd
    [Category] => 14
    [mandator] => Array
        (
            [0] => 1
        )

    [mandator_owner_id] => 1
)

what I am missing. Please help