netbox/netbox/utilities/templates/widgets/markdown_input.html

<div class="border rounded markdown-widget"> 
    <ul class="nav nav-tabs px-3 pt-2 rounded-top border-0">
        <li class="nav-item" role="presentation">
          <button class="nav-link active " id="{{ widget.name }}-input-tab" data-bs-toggle="tab" data-bs-target="#{{ widget.name }}-input" type="button" role="tab" aria-controls="{{ widget.name }}-input" aria-selected="true">
            Write
          </button>
        </li>
        <li class="nav-item" role="presentation">
            <button hx-target="#{{ widget.name }}-preview" hx-swap="innerHTML" hx-post="{% url 'extras:render_markdown' %}" class="nav-link preview-button" id="{{ widget.name }}-markdown-preview-tab" data-bs-toggle="tab" data-bs-target="#{{ widget.name }}-markdown-preview" type="button" role="tab" aria-controls="{{ widget.name }}-markdown-preview" aria-selected="false">
              Preview   
            </button>
          </li>
      </ul>
      <div class="tab-content bg-body rounded-bottom border-top">
        <div class="tab-pane show active" id="{{ widget.name }}-input" role="tabpanel" aria-labelledby="{{ widget.name }}-input-tab">
        {% include "django/forms/widgets/textarea.html" %}
        </div>
        <div class="tab-pane show" id="{{ widget.name }}-markdown-preview" role="tabpanel" aria-labelledby="{{ widget.name }}-markdown-preview-tab">
            <div id="{{ widget.name }}-preview" class="preview px-3 py-2">Testing</div>
        </div>
    </div>
</div>