Use the `required` attribute to mark the element as required. This can be used for form validation purposes.

```html
<div class="max-w-[400px] h-[400px]">
  <sd-select size="lg" label="Required" placeholder="Please select" placement="bottom" value="" required="">
    <sd-option value="option-1">Option 1</sd-option>
    <sd-option value="option-2">Option 2</sd-option>
    <sd-option value="option-3">Option 3</sd-option>
    <sd-option value="option-4">Option 4</sd-option>
    <sd-option value="option-5">Option 5</sd-option>
  </sd-select>
</div>
```