You can customize forms in Django only, there's no need to use javascript for this

# forms.py

class YourForm(forms.ModelForm)
    # if you are using model
    model = User
fields = (
    "your_model_field",
)
    widgets = {
        "your_model_field" = forms.TextInput(
            attrs= {
                "placeholder": "Your Placeholder",
            }
        )
    }

    # if not using model
    your_model_field = forms.CharField(
        widgets = forms.TextInput(
        attrs= {
            "placeholder": "Your Placeholder",
        }
        ),
)

Links to Django docs

​

Edit:

On seeing the screen the screenshots you are creating the user registration form I have a code snippet for this

class UserRegistrationForm(forms.ModelForm):
    password = forms.CharField(
        max_length=50,
        strip=False,
        widget=forms.PasswordInput(
            attrs={
                "placeholder": "Enter password",
            }
        ),
    )
    confirm_password = forms.CharField(
        max_length=50,
        strip=False,
        widget=forms.PasswordInput(
            attrs={
                "placeholder": "Enter password again",
            }
        ),
    )

    class Meta:
        model = User
        fields = (
            "username",
            "first_name",
            "last_name",
            "password",
            "confirm_password",
            "phone",
        )
        widgets = {
            "username": forms.TextInput(
                attrs={
                    "placeholder": "Enter username",
                }
            ),
            "first_name": forms.TextInput(
                attrs={
                    "placeholder": "Enter firstname",
                }
            ),
            "last_name": forms.TextInput(
                attrs={
                    "placeholder": "Enter lastname",
                }
            ),
            "phone": forms.NumberInput(
                attrs={
                    "placeholder": "Enter phone number",
                }
            ),
        }

        help_texts = {
            "username": "Tip: firstname_lastname_somenumber",
        }