Programmatically Assign _customer_user in Woocommerce post meta

I have an external (Non-Wordpress) website that has a table of addresses and associated wordpress user id’s. Each row has a link that opens the Woocommerce Admin page for creating a new order, and populates the billing and shipping information from rawurlencoded $_GET data like this:

http://cooldomain.com/wp-admin/post-new.php?post_type=shop_order&f=BOBBY&l=SMITH
&a=133+M+STREET&b&c=MONROE&s=LA&p=24754&n=US&u=2
You'll notice that the last token is u=2

This is the user_id associated with that address in our WordPress site, so we want to generate the new order for that user.

Here’s the hook we use in functions.php

if (isset($_GET['f'])) {
    add_action('woocommerce_admin_order_data_after_order_details', 'my_auto_populated_meta');
}

Then our function is like this:

function my_auto_populated_meta($order) {
    $first_name = isset($_GET['f']) ? rawurldecode(trim($_GET['f'])) : '';
    ...
    ...
    $uid = isset($_GET['u']) ? intval($_GET['u']) : '';

    update_post_meta($order->id, '_billing_first_name', $first_name);
    ...
    ...
    update_post_meta($order->id, '_customer_user', $uid);
 }

The problem:

When the page first loads and the form is auto-populated, and I echo out the post_meta, the _customer_user value shows as 2, like this:

[_customer_user] = array( [0] => 2 )

But as soon as I click the “Create” button in Woocommerce, it strips that value out so it is:

[_customer_user] = array( [0] => )

How do I make it stick?

Should I use a different action or hook?