How do I install packages in heroku without pip freeze?

See more:

Python

deployment

Heroku

flask

I'm trying to deploy my website with heroku and I'm 90% of the way there. My problem is that when I try to deploy I get an error message that tells me flask_bcrypt isn't found. The reason is because it's not appearing in my requirements.txt file, but I don't know how to add it. I've run pip freeze over and over again but it still doesn't insert flask_bcrypt in.

What I have tried:

I don't know how to work with heroku since this is my first time deploying an app, so does anyone know how to get heroku to install flask_bcrypt and potentially other packages?

Also I get a message saying

sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgres

but I have sqlalchemy in my requirements file, so I don't know why it's not registering it as being there.