advanced-custom-fields

I believe you need the af/email/before_send hook. I found this solution here: https://gist.github.com/mishterk/70bb486baac349c29b684346b77826ce Rather than permanently modifying the email saved with the form entry, you can simply modify the message field to encode the email. For example: <?php // Set the form key you wish to target $form_key = ‘form_5d97cf9edc0a8’; add_action( “af/email/before_send/key=$form_key”, function ( $email, … Read more

Ok got it, for user field actually one has to add “user_” before the id for it to work. So in my case: update_field( $cover_custom_field_key, $cover_id, “user_2” );