Login to the AWS console of the master account under the customized login page, eg. https://123456.signin.aws.amazon.com/console. This should require you to enter your username/password and your MFA token to login.
Go to AWS Single Sign-On in the service list. This can be done under https://eu-west-1.console.aws.amazon.com/singlesignon.
Click on the Enable AWS SSO button.
Click on Directory in the sidebar.
Click on Change directory in the Directory section to switch to the AD Connector.
Click on Directory in the sidebar.
Click on Customize in the User portal URL section to define a custom URL.
Please be aware that you won’t be able to change this later.
TBD