Using authenticated user from external application

I am an external application that needs to log in MongoDB using Ream Auth and then access data afterwards in the app using a web api.
I created a web api that authenticates the user and returns the user object that is returned upon app.logIn(credentials)
In another screen of my external app, I now need to access data from MongoDB using the authenticated user.
I will do something like this in my API
mongodb = app.currentUser.mongoClient(“mongodb-atlas”);
How do I set the currentUser to be equal to the returned user object without having to get the credentials again?