Need add one Custom field

Good afternoon. The question often, but I could not find an answer. I need on the page add / edit invoice (wp-admin/admin.php?page=wpi_page_manage_invoice) add one custom field. Tell me how I am ready even in the code to do just Explain how. I tried to do as it says here (https://codex.wordpress.org/Custom_Fields) , but to no avail.

https://wordpress.org/plugins/wp-invoice/