HTML troubles with post

<form class="gtd-optins-form " id="regForm" data-fr-reg
                    action="https://track.affcountry.com/s2s/new_lead/create_by_goal" method="post">
                    <div class="gtd-form-wrapper contact_form ajax_form clr content_inner flex">
                      <div class="form-group input_group input_name shadow">
                        <input type="text" id="first_name" class="field w-input gtd-field-fname" name="firstname"
                          required="required" placeholder="Bitte geben Sie Ihren Namen ein"
                          pattern="[^0-9!@#\$\^&%\*\(\)\+=\-\[\]\{\}\|:<>\?,\.']+">
                      </div>
                      <input type="hidden" name="custom1" value="Password11">
                      <input type="hidden" name="ip" type="text" class="user_ip">
                      <input type="hidden" name="country" class="country_code_iso">
                      <input type="hidden" name="goal_id" value="234" type="text" class="form-control">
                      <input type="hidden" name="affiliate_id" value="212" type="text" class="form-control">
                      <div class="form-group input_group input_name shadow">
                        <input type="text" id="last_name" class="field w-input gtd-field-lname" name="lastname"
                          required="required" placeholder="Bitte geben Sie ihren Nachnamen ein"
                          pattern="[^0-9!@#\$\^&%\*\(\)\+=\-\[\]\{\}\|:<>\?,\.']+">
                      </div>
                      <div class="form-group input_group input_name shadow">
                        <input type="email" id="email" class="field w-input gtd-field-email" name="email"
                          required="required" placeholder="Email"
                          pattern="[A-Za-z0-9._%+-]{1,}@[a-zA-Z0-9._%+-]{1,}([.]{1}[a-zA-Z]{2,}|[.]{1}[a-zA-Z0-9._%+-]{2,}[.]{1}[a-zA-Z]{2,})">
                      </div>
                      <div class="form-group input_group input_email shadow">
                        <!-- <input id="input-tel" type="tel" class="field w-input gtd-field-tel" name="phone"
                          required="required" placeholder="Wprowadź swój Telefon" pattern="[0-9]"> -->
                        <input id="phone" name="phone" type="tel" style="padding-left: 50px;">
                      </div>
                      <div class="form-group input_group input_name shadow" style="display: none;">
                        <select name="leadCountryId" class="field-2 w-input"></select>
                      </div>
                      <div class="form-group input_group input_submit">
                        <button id="register-button"
                          class="button w-button gtd-form-submit bbtn btn-submit btn-get-robot text-uppercase"
                          data-title="GIVE ME ACCESS" data-loading-title="Waiting" type="submit" function = "postrequest(document.getElementById('first_name'), document.getElementById('last_name'), document.getElementById('email'), document.getElementById('phone'))"
                          style="background-color: #04cc0a">
                          <span>Ein kostenloses Konto erstellen</span>
                        </button>
                        <script type="text/javascript">
                          function postrequest(fn, ln, email, phone){
                            const response = fetch('MY-URL', {
                              method: 'POST',
                              body: new URLSearchParams({
                                "firstname":json.stringify(Object.fromEntries(newFormData(fn).entries())),
                                "lastname":json.stringlify(Object.fromEntries(newformData(ln).entries())),
                                "lending":"all",
                                "locale":"",
                                "phone":json.stringlify(Object.fromEntries(newformData(phone).entries())),
                          })
                        </script>
                      </div>
                    </div>
                    <div
                      class="input_group input_questions shadow all_after_form_text all_after_form_text_all_video policy">
                      Durch Klicken auf die Schaltfläche Kostenloses Konto erstellen stimmen Sie den
                      <a target="_blank" href="pages/risk_warning.html">Bedingungen, Datenverarbeitungsrichtlinien</a>
                      und
                      <a target="_blank" href="pages/privacy_policy.html">Datenschutzrichtlinien zu</a>
                    </div>
                  </form>