How to add CSS class names to forms?

I created the form using {{ form:create }} tag, as in:

{{ form:create in="contact" }}

How do I specify the class names, which are going to be rendered in the HTML. The end result for the opening <form> tag should be:

<form method="POST" action="/!/Form/create" class="flex flex-wrap">
Answered by Amadeusz Annissimo!
>>>>>>> Answered <<<<<<<
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