Hi,
There is no setting for it, the target is written directly in the view file of the product page.
Your product pages use the "Tabular" layout, so the file to change is product / show_tabular.php. Go in the HikaShop backend, menu Display > Views, search for show_tabular, and click on the file to edit it. Look for the line containing MANUFACTURER_URL :
echo \Joomla\CMS\Language\Text::sprintf('MANUFACTURER_URL', '<a href="' . $this->element->product_url . '" target="_blank">' . $this->element->product_url . '</a>');
Remove target="_blank" from that line and save. Saving from that interface creates a view override in your template, so the change will not be lost when you update HikaShop.
Note that each product page layout has its own file, so if you later switch the "Layout on the product page" setting of the HikaShop configuration to another layout, the same change has to be made in the file of that layout.