Didn't get 100$ promotional credit

Under 'Account' -> 'Billing Info' there is an 'Add a promo code' link when the account is less than 90 days old.

Reference: https://www.linode.com/docs/guides/manage-billing-in-cloud-manager/#adding-a-promo-code

Yeah i see the 'Add a promo code', but how can I get a code?

Easiest way I know of is to go to one of the documentation pages while not logged into your account and click on the 'try this guide' bit at the top of a doc. The page it takes you to usually shows the code for that on it somewhere.