Вы можете использовать это:
forms.py
class RegForm(forms.ModelForm):
class Meta:
model = User
widgets = {
'username':forms.TextInput(attrs={'class':'form-control mb-4','required':True,'placeholder':'Username'}),
'email':forms.EmailInput(attrs={'class':'form-control mb-4','required':True,'placeholder':'Email'}),
'password':forms.PasswordInput(attrs={'class':'form-control mb-4','required':True,'placeholder':'Password'}),
}
fields = ('username','email','password',)
html
{{ form.username }}
{{ form.email }}
{{ form.password }}
{% csrf_token %}
<button class="btn btn-info mt-3 btn-block waves-effect waves-light" type="submit">Register</button>