JSFiddle - React, Tailwind, and code Playground

HTML

<form name="phpmailerServerSettings" method="POST" action="" role="form">
    <div id="form-inputServer" class="form-group">
    <label for="inputServer">Mailudbyder</label>
    <input type="text" name="inputServer" class="form-control" value="smtp.example.com">
    </div>
    <label for="mail_auth">Min server kræver godkendelse</label>
    <input type="checkbox" name="mail_auth" id="auth_mail" class="form-control " value="auth_yes" onchange="mail_auth_check()">
    <div id="form-inputUsername" class="form-group">
    <label for="inputUsername">Brugernavn</label>
    <input type="text" name="inputUsername" id="inputUsername" class="form-control" value="[email protected]">
    </div>
    <div id="form-inputPassword" class="form-group">
    <label for="inputPassword">Adgangskode</label>
    <input type="password" name="inputPassword" id="inputPassword" class="form-control" value="password1234">
    </div>
    <div id="form-inputSendFromName" class="form-group">
    <label for="inputSendFromName">Navn</label>
    <input type="text" name="inputSendFromName" class="form-control " value="Website navn">
    </div>
    <div id="form-inputSendFromAddress" class="form-group">
    <label for="inputSendFromAddress">Send fra adresse</label>
    <input type="text" name="inputSendFromAddress" class="form-control" value="[email protected]">
    </div>
    <div id="form-inputSSL" class="form-group">
    <label for="inputSSL">Min server kræver SSL kryptering</label>
    <select name="inputSSL" multiple required>
    <option value="ssl">Ja (SSL)</option>
    <option value="tls">Ja (TLS)</option>
    <option value="no" selected="">Nej</option>
    </select>
    </div>
    <div id="form-element-actions" class="form-group">
    <button name="phpmailerServerSettings" type="submit" class="btn btn-rad btn-primary">Gem indstillinger</button>
    </div>
    </form>

JavaScript

$(function(){
    $("#inputUsername, #inputPassword").attr("disabled", 1);
     $("#auth_mail").change(function(){
         if($(this).is(':checked')) {
             $("#inputUsername, #inputPassword").removeAttr("disabled");
         } else {
             $("#inputUsername, #inputPassword").attr("disabled", 1);
         }
     });
 });