Hi,
In the canonical URL field, you can enter a relative URL.
So instead of
https://www.hikashop.com/support/forum.html
, you would enter just
.
So provided that the menu structure is the same for all the websites, the canonical URL will get the correct domain name automatically prepended to it and the canonical URL should be ok on all the websites.
I suppose the hikashop_product table could be duplicated per website in MightySites so that you would have one canonical URL per product per website, but then you would have to keep track of the product information across the websites and it would be problematic if you need stock management on them as the product_quantity column wouldn't be shared anymore.
So I would recommend to check with MightySites what could be done knowing all what I wrote above.