All form controls need to be properly labeled.

The current rendered markup for an embed url field is:

<form>
  <input type="url" value="" class="components-placeholder__input" placeholder="Enter URL to embed here…">
  <button type="submit" class="components-button button button-large">Embed</button>
</form>

Visually:

One option could be to simply use the visible text (in the example above: "Cloudup URL") as label for the input field.
Worth reminding, according to the WordPress accessibility coding standard, labels should be explicitly associated (use for/id attributes and not wrapping the field). See https://make.wordpress.org/core/handbook/best-practices/coding-standards/accessibility-coding-standards/#labeling