_preview_button_on_edit.html

<input type="button" class="action-preview button"
    data-action="{% url 'wagtailadmin_pages_preview_on_edit' page.id %}{% if mode %}?mode={{ mode|urlencode }}{% endif %}"
    data-placeholder="{% url 'wagtailadmin_pages_preview_placeholder' %}"
    data-windowname="wagtail_preview_{{ page.id }}" value="{{ label }}" />