index.html
<!DOCTYPE html>
<html>
<body>
<h1>placeholder</h1>
<form action="/test.html">
<label for="phone">Enter a phone number:</label><br><br>
<input type="tel" id="phone" name="phone" placeholder="123-45-678" pattern="[0-9]{3}-[0-9]{2}-[0-9]{3}"><br><br>
<small>Format: 123-45-678</small><br><br>
<input type="submit">
</form>
</body>
</html>