WooCommerce Login From External Site

Hi
I have built a site that requires users are automaticaly logged in to WooCommerce.

The customers start the process on a portal site, choose a link to my site and are redirected. Password and Username plus several other hidden fields, such as unique custom ref and a return URL, are posted to my login page, these must be processed, the user logged in and redirected to the main product page. (The redirection is no issue).

At the end of the process when the order is made, the cart info and user are sent back to the portal site. I can redirect back to the portal site with all the relevant info at the end of the process, but cannot figure out how to capture the initial data and auto log in.

Any advice would be greatly apprecieted.

https://wordpress.org/plugins/woocommerce/