Question: We are looking to access an external vendor's Redshift data. We are on the same platform (AWS). What is the best approach?

Is it possible to directly connect to their Redshift instance as opposed to copy data from their Redshift database to an S3 bucket in our account?

Answer: 
I have solved your same problem changing the user database password with a password like name.number in the database access in the security section. Then copying that password in the MongoURI
in your database URI You got to replace with username that you created on mongodb Atlas same goes with the and .
For example if your credentials are:-
username: jonDoe, password: test@abcd, database name: myBlogs

mongodb://jonDoe:test@abcd@xxxx-uaglu.mongodb.net/myBlogs?retryWrites=true&w=majority.