from cakemail.models import ScheduleCampaign

api.campaign.schedule(
    campaign_id=<campaign ID>,
    schedule_campaign=ScheduleCampaign(
        date=<your scheduled date>
    )
)

<?php

$api->campaign->schedule([
    'campaign_id' => <campaign ID>,
    'schedule_campaign' => new \Cakemail\Lib\Model\ScheduleCampaign([
        'date' => <your scheduled date>
    ])
]);