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HTML

<div>
     Extension:<br>
     <input type="text" id="extension" name="extension" placeholder="Please enter phone ext's" required>
   </div>
   <!-- JavaScript file include -->
   <script type="text/javascript" src="js.js"></script>

JavaScript

//ammend field function
let ammend_field = () => {
 //get element by id
 let ext = document.getElementById("extension");

 //use regex match to replace ext.value input field
 //original answer from:
 //http://stackoverflow.com/questions/17260238/how-to-insert-space-every-4-characters-for-iban-registering
 ext.value = ext.value.replace(/[^\d0-9]/g, '').replace(/(.{4})/g, '$1 / ').trim();
}

//add event listener to submit button
document.getElementById("extension").addEventListener("keypress", ammend_field);