<script type="text/javascript">
        $(document).ready(function () {
            $("#field_et353").keyup(function () {
                var value = $(this).val();
                $("#field_dfg345").val(value);
            });
        });
</script>

Start your code here
****test1
<script type="text/javascript">
        $(document).ready(function () {
            $('select[name="item_meta[336]"]').keyup(function () {
                var value = $(this).val();
                $("#field_ehb8a").val(value);
            });
        });
</script>
TEST2
<script type="text/javascript">
        $(document).ready(function () {
            $("#field_gsl0z").keyup(function () {
                var value = $(this).val();
                $("#field_ehb8a").val(value);
            });
        });
</script>

****try also all kind
Text Input (date, number, etc...):
#field_key
Dropdown:
select[name='item_meta[994]']
Multiple Select Dropdown:
select[name='item_meta[994][]']
Radio Button:
input[name="item_meta[988]"]
Checkbox/Toggle:
input[name='item_meta[453][]']



The source field are appointment field https://formidableforms.com/knowledgebase/simply-schedule-appointments-forms/#kb-download-and-install

Start your code here

<label for="field_gsl0z" id="field_gsl0z_label" class="frm_primary_label control-label">BOOKING 
        <span class="frm_required" aria-hidden="true">*</span>
    </label>

<input type="hidden" id="field_gsl0z" name="item_meta[336]" value="26721" data-reqmsg="Questo campo non può essere vuoto." aria-required="true" data-invmsg="BOOKING RAPALLO non è valido" class="ssa_appointment_form_field_appointment_id form-control ng-pristine ng-untouched ng-valid ng-empty" ng-model="BOOKING" aria-invalid="false" autocomplete="off">