WooCommerce Booster自定义输入字段 - 通过PHP获取价值

I want to set up a third party API that fires up after a payment is completed. Like this: https://www.mitchcanter.com/woocommerce-third-party-api-calls/

I'm using the WooCommerce Booster plugin to get a custom field where the customer can enter a string.

So my question is, how do I access that string via PHP to use it in my API call?

I can't seem to find it using WC_Order, or WC_Meta_Data... the meta_key is "_wcj_product_input_fields_local_1"...

Can anyone help me with this please?