To link your itsme to your user id, you will need to authenticate with a Isabel card or IsaKey. This is needed when you link for the first time, you change phone or phone number.

If you have more than one user that you want to use on this PC, you can add another ones.
Click the "Add a new user" button and login with the IsaKey of this user. After a successful login, the user will be saved locally on this PC.

The One-Time Activation Code is a code that provide you access to the IsaKey activator. Be sure that you followed the steps on the letter.
Not following the steps will result in failure of the code. Read the document attentively.