Using OAuth 2.0 is necessary for developing apps that are available in the Pipedrive Marketplace. Authorization via OAuth 2.0 is a well-known and stable way to get fine-grained access to an API. To retrieve OAuth2 tokens you should send requests to the https://oauth.pipedrive.com domain. After registering the app, you must add the necessary server-side logic to your app to establish the OAuth flow. Please read more about authorization step on the Pipedrive Developers page.