from cakemail.models import SendTestEmail

api.campaign.send_test_email(
    campaign_id=<campaign ID>,
    send_test_email=SendTestEmail(
            email='<test email address>',
            type='merged'
    )
)

<?php

$api->campaign->sendTestEmail([
    'campaign_id' => <campaign ID>,
    'send_test_email' => new \Cakemail\Lib\Models\SendTestEmail([
        'email' => '<test email address>',
        'type' => 'merged'
    ])
]);