Is it permissible to register on a website that sells dietary supplements, some of which are not slaughtered Islamically, in order to benefit from discounts, while avoiding the purchase of what is forbidden? And is registration considered assisting the website in spreading forbidden things and supporting it?

There is no objection to registering on a site like the one mentioned to purchase permissible goods. This is not considered prohibited assistance, for permissible dealings with those who have impermissible dealings are not forbidden by Sharia. Muslims have dealt with polytheists through buying and selling since the time of the Messenger of Allah, peace and blessings be upon him. The evidence for this is that "the Messenger of Allah, peace and blessings be upon him, gave Khaybar to the Jews to work and cultivate, and they would have half of what it produced," and "the Messenger of Allah, peace and blessings be upon him, bought food from a Jew on credit and gave him his shield as collateral."