Custom Form

I have custom forms on my websites and i would like show the consent privacy-policy checkbox.
I’ve put it by myself by doing this :

<div class="custom-control custom-checkbox">
<input type="checkbox" class="form-control custom-control-input" id="privacy-policy" name="privacy-policy">
<label class="custom-control-label" for="privacy-policy"> J’accepte <a href="https://sba.aedesit.com/charte-de-vie-privee__trashed/">les conditions et la politique de confidentialité</a>.</label>
</div>

and in my php code this :

if (isset($_POST['privacy-policy']) && $_POST['privacy-policy'] === 'on') {
$dataSubject = gdpr('data-subject')->getByEmail($_POST['email']);
$dataSubject->giveConsent('privacy-policy');
}

but this returns me an error :
Class gdpr.data-subject does not exist

Can you tel me the correct method to implement this to a custom contact form ?

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