Simple Affiliate Plugin - Moved Shop/Accounts to Seperate Install

Hi

I just recently separated my shop and other client services (account management, invoices, support tickets) to a seperate WordPress install (subdomain) to the actual main website. This has increased the speed of my main site massively.

Previously I had a referral system, so users could refer clients to the website and anyone that orders or makes an account on the site would gain commission.

The issue now is, the referral system won't work since all client features are on the other install, including the referral plugin for WooCommerce etc.

So, what I'm looking for is to have a very simple referral plugin where I can store the same referral code on the main website, then everyone on the main site I link to the "client/shop" area, I also add that referral code onto the URL automatically, so it will then add them as a referral in that system.

I could probably figure this out over some time, but wondered if anyone knows of some simple plugins for this, or has some code ready made.

Thanks!