Affiliate partner customers aren't detected as such!

-- Joomla version -- : 3.8.3
-- PHP version -- : 5.8.35

We have an affiliate partner. We have his link identified by ?partner_id=xxx as incoming list. During checkout, the shop looses the partner_id and process goes as usual just like an ordinary customer and the shop doesn't list them under partners orders.
Also, we added a coupon so the partners customers can add during checkout but there is no way on forcing them to use or usually they forget since the coupon doesn't contain any benefit (its partners decision). If we use 'force coupon on checkout', it loads for every customer and that is not a solution.

How can we solve this problem?!
(The hikashop version is 3.2.2)