nk20/templates/note/amount_input.html

<div class="input-group">
    <input class="form-control mx-auto d-block" type="number" min="0" step="0.01"
           {% if widget.value != None and widget.value != "" %}value="{{ widget.value }}"{% endif %}
           name="{{ widget.name }}"
            {% for name, value in widget.attrs.items %}
                {% ifnotequal value False %}{{ name }}{% ifnotequal value True %}="{{ value|stringformat:'s' }}"{% endifnotequal %}{% endifnotequal %}
            {% endfor %}>
    <div class="input-group-append">
        <span class="input-group-text">€</span>
    </div>
    <p id="amount-required" class="invalid-feedback"></p>
</div>