Anyone with a premium Sales Navigator has a referral option in the top right corner. You can send an invitation there, thanks to which your friend gets the premium version free for two months.
This only applies if that person hasn't sent more than ten such referrals. And it doesn't apply if the recipient has used any other free premium service in the last 12 months.
In any case, remember that even the free premium version will require you to enter a payment card, and if you don't cancel it in time, you'll be charged starting from the third month.
It's a nice way to save about 200 EUR and find out how powerful Sales Navigator Core is when used correctly.
Bonus trick: cancel the premium version after 58 days. LinkedIn will ask you why you're canceling, and you say it's too expensive. They'll offer you another two months at a 50% discount.
If I were the owner of LinkedIn, I'd do the same thing. This premium pays off for most demanding users, you'll get used to it, and you'll be fine with its price. Then you'll buy an annual subscription at another 33% discount.
Note: This article describes the most common behavior of this network as of August 2023. However, different rules may apply to your account.