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>