If your are using the Bitly API, you will need your Oauth Access Token. To find your OAuth access token navigate to the hamburger menu button in the top right hand corner. From the drop down select your account tab. Navigate to the 'Generic Access Token' tab.
If you need to generate an access token so you can use our API, you can go to https://bitly.com/a/oauth_apps, click "Generic Access Token",
Then enter your password to create the token, and then copy that token into your code or configuration.
Access tokens don't expire (though you can expire them manually if yours leaks into the wild), so you can use the single access token as long as you need.