from cakemail.models import CreateCustomAttribute

custom_attribute = api.custom_attribute.create(
    list_id=<list ID>,
    create_custom_attribute=CreateCustomAttribute(
        name='<attribute name>',
        type='<attribute type>'
    )
)

<?php

$customAttribute = $api->custom_attribute->create([
    'list_id' => <list ID>,
    'create_custom_attribute' => new \Cakemail\Lib\Model\CreateCustomAttribute([
        'name' => '<attribute name>',
        'type' => '<attribute type>'
    ])
]);