<div class="input-field col-md-12">
<div class="row">
{{ form.username }}
<label for="nick_name">Нікнейм</label>
</div>
</div>
<div class="input-field col-md-6">
{{ form.first_name }}
<label for="first_name">Ім’я</label>
</div>
<div class="input-field col-md-6">
{{ form.last_name }}
<label for="last_name">Прізвище</label>
</div>
self.fields['field_name'].widget.attrs['class'] = 'your_class'