Root User credentials for member account in an AWS Organization

I created an AWS account (A). Used account A to create an AWS Organization. Created a new AWS account (B) from the AWS Management Console inside the organisation. While creating the new account B, it only asked me account name & email.

Given this, how do we log into account B directly (without Role switching from account A)? I was never asked to set a root user password. Am I missing something simple?