Easiest way to achieve this is to update your post title using wp_update_post AFTER your acf_form() array – saves all the headache of creating a pre_save_post function and whatnot.
Example:
$current_user = wp_get_current_user();
$current_user_id = $current_user->ID;
?>
<?php acf_form([
'post_id' => 'new_post',
'field_groups' => [19689],
'html_before_fields' => '<div id="formContainer">',
'html_after_fields' => '</div>',
'uploader' => 'basic',
'submit_value' => 'Submit',
'updated_message' => false,
'new_post' => array(
'post_type' => 'custompost',
'post_status' => 'publish',
)
]); ?>
<?php
//update post title using WP_Query, getting relevant post meta and passing it to the wp_update_post() function
$updatePost= new WP_Query([
'post_type' => 'custompost',
'post_status' => 'publish',
'author' => $current_user_id
]);
//loop
if($updatePost->have_posts()){
while($updatePost->have_posts()){
$updatePost->the_post();
$newTitle = get_post_meta(get_the_ID(),'field_name', true );
if($jobTitle){
wp_update_post([
'post_title' => $newTitle
]);
}
}
wp_reset_postdata();
}
?>
