My form works but do not show success message/alert

I have a website
My register form work but do not show success message/alert. I tried adding a with the status that should change display=“block” or “none”, tried alert after the fetch, but no luck. So people dont know that they are registered

My HTML Preformatted textcode for the form is:

   <form name="register-form" class="register-form" method="post" netlify action="/">
                <div class="form-field">
                    <label for="name-parent">Име родител:</label>
                    <input class="form-input-field" type="text" name="name-parent" id="name-parent" required="true">
                <div class="form-field">
                    <label for="email">Имейл адрес:</label>
                    <input class="form-input-field" type="email" name="email" id="email" required="true">
                <div class="form-field">
                    <label for="phone">Телефонен номер:</label>
                    <input class="form-input-field" type="text" name="phone" id="phone" required="true">
                <div class="form-field">
                    <label for="activity">Желая да регистрирам детето/децата си в: </label>
                    <select name="activity" id="activity" onchange="onSelect()">
                        <option value="Dve-Kalinki-spring">Клуб "Две Калинки" - пролетен срок</option>
                        <option value="Dve-Kalinki-1-visit">Клуб "Две Калинки" - еднократно посещение</option>
                        <option value="fika">Фика</option>

                <div class="form-field " id="date-field">
                    <label for="date">Дата за еднократно посещение: </label>
                    <input class="form-input-field" type="text" name="date" id="date" value=" ">

               <div class="form-field">
                    <label for="kids">Моля напишете името/имената и възрастта на детето/децата:</label>
                    <textarea name="kids" id="kids" cols="30" rows="5"></textarea>
                <button class="btn" id="registerBtn" type="submit">Регистрация</button>
               <p class="formStatus"></p>

My JavaScript code:

const onRegister = (event) => {
    const myForm =;
    const formData = new FormData(myForm);
    fetch("/", {
      method: "POST",
      headers: { "Content-Type": "application/x-www-form-urlencoded" },
      body: new URLSearchParams(formData).toString(),
      .then(() => alert("Thank you for your submission"))
      .catch((error) => alert(error));

  .addEventListener("submit", onRegister);

You’ve got this form on your website as well:

Since querSelector looks for the first match, the event listener is added to that form. Change the way to select the correct form.