如何在select中选择一个选项后在输入字段中自动生成代码 - PHP [重复]

Question

This question already has an answer here:

• Populate one dropdown list based on another dropdown list 3 answers
• How to populate second <select> according to first <select> option 2 answers
• Populate one dropdown based on selection in another 3 answers

I have this html code:

<select class="form-control required" id="dprt" name="dpro_tipo" onchange="displayDprtForms();">
   <option></option>
   <?php foreach ($this->dprts as $dprt) { ?>
       <option name="tipo-select" value="<?= $dprt->dprt_cod_doti ?>"><?= $dprt->dprt_descri ?></option>
   <?php } ?>
</select>

<div class="form-group form-reg">
   <input value="<?php echo $this->barcodegen?>" name="dpro_barcode" class="form-control barcode-autogen">
</div>

When i load the page the fist option for the select is empty so the carcode has to be an empty input field but when i change the option on the select has to be auto generated. I already have a query that works good for the auto generation and the js to get the next code based on the option. What i cant do is an ajax call or something like that for generate the code only when in the select field there is something. I hope the question is clear enough. thanks

</div>