JSFiddle - React, Tailwind, and code Playground

HTML

<form>Email:
    <input type="text" name="email">
    <br>
        <label>Hidden input: (visible for example purposes)</label><input name="retype-email">
    <br>
    <input type="submit" value="Submit">
</form>

JavaScript

document.forms[0].addEventListener('submit', function(){
    document.querySelector('[name="retype-email"]').value =  document.querySelector('[name="email"]').value;
});