Step 1:

Create a Custom Field and name it masked_input

Step 2:

Add a new Custom Page and enter the code below

<div>
  <h2>Enter Social Security Number</h2>
  <input name="masked_input" id="masked_input" />
  <br />
  <br />
  <button type="submit" class="btn next-btn pure-button pure-button-primary">Next</button>
</div>

<script>
  let maskedInput = document.getElementById('masked_input');
  maskedInput.type = 'password';
  maskedInput.placeholder = 'Social Security Number';
  maskedInput.required = true;
</script>