If you're asking how to receive a referral code that gives you/the referrer an extra month free, you're welcome to use mine, or anyone else's who comments: https://ynab.com/referral/?ref=4DNbvAp0Of5-Xf_T&utm_source=customer_referral . You can likely also find referral codes online via a Google search. I know the YNAB subreddit on Reddit occasionally has a topic with referral codes that can be used.
If you want to know how to provide a referral link to someone else (and have subscribed through the website, not through iTunes), there should be a "Share YNAB. Get YNAB Free" link in the bottom left corner of your screen when you're logged into YNAB, right above your email address. Click on it, and you should get taken to https://app.youneedabudget.com/referral_program , which gives you sharing options and the link you'll need to provide to the person you want to refer.