<head>
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr/dist/flatpickr.min.css">
  <script src="https://cdn.jsdelivr.net/npm/flatpickr"></script>
</head>

        <div class="field">
          <label for="ContactFormDates">Wunschtermin(e)</label>
          <input
            type="text"
            id="ContactFormDates"
            name="contact[Wunschtermine]"
            placeholder="Datum auswählen"
          >
        </div>
        <script>
          document.addEventListener("DOMContentLoaded", function () {

            flatpickr("#ContactFormDates", {
              mode: "multiple",
              dateFormat: "Y-m-d",
              minDate: "today"
            });

          });
        </script>