from cakemail.models import List, Sender

list = api.list.create(
    list=List(
        name='<list name>',
        language='en_US',
        default_sender=Sender(id='<sender ID>')
    )
)

<?php

$list = $api->list->create([
    'model_list' => new \Cakemail\Lib\Model\ModelList([
        'name' => '<list name>',
        'language' => 'en_US',
        'default_sender' => new \Cakemail\Lib\Model\Sender([
            'id' => '<sender ID>'
        ])
    ])
]);