How to pass UTM params through Form

My Mautic version is: v2.15.3
My PHP version is: 7.2

My problem is:
I have the Form in my opt-in page. Submit Form action is redirect to page2. I need to pass UTM params when this redirect occurs. How is it possible?

For example user land on opt-in page/?utm_source=something_dynamic, and then fills and submit the Form, i want him to redirect to page2/?utm_source=something_dynamic.
When i used getresponse before Mautic, i made this by adding php code to link which catch UTM parameter, like this:


  <input type="hidden" name="thankyou_url" value="<?=@$_GET['utm_source']?>"/>


Form action is configuring in Mautic inteface, and when i paste this link “<?=@$_GET['utm_source']?>” in Redirect URL and press save, it didn’t save php code (screen:

I also tried to choose “Remain at Form” in Mautic, and then add this code manually to my Form:

<input type="hidden" name="thankyou_url" value="<?=@$_GET['utm_source']?>"/>

But redirect didn’t working that way.

Please help