Check out example codes for "html input placeholder". It will help you in understanding the concepts better.

Code Example 1

<form action="/action_page.php">
	<label for="phone">Enter a phone number:</label>
    <input type="tel" id="phone" name="phone" placeholder="123-45-678" pattern="[0-9]{3}-[0-9]{2}-[0-9]{3}"><br><br>
 </form> 

Code Example 2

placeholder="123-45-678"

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