Evening Joost,
I suppose that -> registreer voor webshop is the temporary menu item you created, if you open your menu manager and look at the menu item ID of the temporary menu is that the same ID you did use for the Hikashop Registration Redirect Plugin?
If not, set the the (correct) temporary menu ID in the Hikashop Registration Redirect Plugin and your normal registrationlink should work, so check this first and test it.
Since you do not want the extra menu item for your registration form showing on the site, you could make it a hidden menu item. There is already a standard HikaShop hidden menus in your menu manger, so you could creat a new registration menu link there.
Regards, Rene