Hi @jacquesmivi
If you want to redirect it to the newly created post, kindly use the %post_url% tag like this:
'return' => %post_url%,
This page should give you more idea about it: https://www.advancedcustomfields.com/resources/acf_form/.
If you need to generate the link based on the newly created post ID, then I suggest you do the redirection by using the wp_redirect() function in the acf/save_post action instead.
I hope this helps 🙂
