map the value of custom field into wp default post_titel

So if i stand correctly, you want the field with the name Headline to be the post title?
If so.

Edit your post type within metabox
Go to Supports
Make sure Title is turned of

Then create a code snippit (download first a snippit plugin)

function change_post_title_by_field_value( $post_id ) {
    if ( get_post_type( $post_id ) == 'YOUR_CUSTOM_POST_TITLE_HERE') {
        $headline_title= rwmb_meta( 'headline', $post_id );
        $title = $headline_title;
        $title = sanitize_text_field( $title );
        $new_slug = sanitize_title( $title );
    } else {
        return;
    }
    
    // prevent a loop
    remove_action('save_post', 'change_post_title_by_field_value');
    
    $content = array(
        'ID' => $post_id,
        'post_title' => $title,
        'post_name' => $new_slug,
    );
    wp_update_post($content);
    
    add_action('save_post', 'change_post_title_by_field_value');
    
}
add_action('save_post', 'change_post_title_by_field_value');

what is the name from your custom post type?