How do I deploy a Flask app using a MongoDB database to Heroku?

# obtain connect string from Atlas
connect_string = 'mongodb+srv://user:pwd@mycluster-ubiho.azure.mongodb.net/mydb?retryWrites=true&w=majority'

client = MongoClient(connect_string)
db = client.get_default_database()
# query users collection
db.users.find_one({"user":"beppe"})