Add a custom validator to Parsley (which works unlike the docs) From https://docs.typo3.org/typo3cms/extensions/powermail/ForDevelopers/WriteOwnValidators/ClientsideValidation/Index.html

parsley.multipleof.js

<input type="text" data-parsley-multiple="3" data-parsley-error-message="Please try again" />
    /*...*/
<script type="text/javascript">
    window.ParsleyValidator
        .addValidator('multipleof', function (value, requirement) {
            return 0 === value % requirement;
        }, 32)
        .addMessage('en', 'multipleof', 'This value should be a multiple of %s');
</script>