There are differences between a field label and a placeholder. A placeholder as in HTML’s placeholder attribute must not replace a <label> element, because this is necessary for accessibility. The placeholder attribute should only contain explanatory text that helps convey the intended content of the field. What you can do is use CSS (and possibly JavaScript) to make the label look like a placeholder (as demonstrated on https://material.io/components/text-fields). Alternatively (although not recommended), you can hide the label by positioning it outside the viewport using CSS and add the label text to the placeholder attribute with the 'form_attributes' => array() parameter as described in the documentation.
