mongo -u username -p password
use admin db.grantRolesToUser( "myUsersName", [ { role: "root", db: "admin" }, ] );
References
https://stackoverflow.com/questions/27222279/mongodb-user-roles
https://www.mongodb.com/docs/manual/reference/built-in-roles/
https://www.mongodb.com/docs/manual/reference/method/db.grantRolesToUser/