Environment variables
You can set environment variables for a project by adding a special type of Integration. Go to Integrations in the right-hand sidebar, and then either modify an existing "Environment variables" integration or add a new one:
Environment variables are a secure way to store secrets. They are stored encrypted, and decrypted only before mounting them into your project.
Don't forget to connect your project(s) to the newly created "Environment variable" from within the project itself. Once added to the project, you can easily use the Environment Variable from within your notebook:

Connecting to any service or integration

You might be storing your data in a database that does not yet have a native integration in Deepnote. Or perhaps you need to use a third-party library that issues an API key (e.g. Mailgun).
In this case, you can add an environment variable which will let you access the remote integration or third party service. An example is Deepnote's neptune.ai integration.
Some integrations can't be used just by including environment variables. Let us know if you are looking to use a service like this in Deepnote.