Activate your itsme account via this link or click on the "My itsme account" tab in your customer account settings. To access your customer account settings, log in to your Customer Account and click on the arrow next to your meter number at the top right of the screen. You will then need to enter the national register number(s) that will be used the next time you log in via itsme.
The next time you log in to your Customer Account, you can click directly on "Log in with itsme".