JSFiddle - React, Tailwind, and code Playground

HTML

<label for="flat_name">Flat Name</label>
<input type="text" name="flat_name" id="flat_name" />
<label for="mobile_username">Generated Text</label>
<input type="text" name="mobile_username" id="mobile_username" value="qwerty_" disabled="disabled" />

JavaScript

$('#flat_name').keyup(function () {
    $('#mobile_username').val += ($(this).val());
});