How do I set up a .env on Heroku for my production database?

I am currently building an application in Laravel and recently deployed it to Heroku. Just as I have an .env file, my guess is I should have one as well on my server to help with my database.

How can I do that on Heroku? Most of the thing refer to adding APP_ENV to the config, but that's just for the key. I am not sure what to put in the value field.