The acf_form function allows you to define some custom HTML. Please see the docs to read more on this.
I would add some custom HTML as a hidden input with a name of ‘my_post_type’. Please don’t use ‘post_type’ – this will conflict with WP.
Then in your acf/pre_save_post filter, you can create a new post and use the ‘my_post_type’ value in the $_POST to set the post type.
Hope that helps.