Saving custom post in custom form

This is not really an answer but more an explenation to how stuff works.

What do you mean no post id?
Think about it, you want to
update a post meta but which post?

If you are trying to create a new post you need a different code.
if you want to update a specific post you must tell wordpress
which one to update meaning you must attach a post ID.

update_post_meta function clearly states that the ID is required.

Since i am pretty sure you actually want to create a new post
from scracth and not update an exisiting post here is an example
how you can do that.

$post = array(
    'post_title'    => 'some title',
    'post_status'   => 'publish',
    'post_type' => 'custom_post_name'
);

$new_post_id = wp_insert_post($post, 10, 1);

// Do the wp_insert_post action to insert it
do_action('wp_insert_post', 'wp_insert_post', 10, 1); 
update_post_meta($new_post_id, 'some_meta_field', 'some_data');