From the 16th of November 2017 myCloud is also supporting Swisscom Login.
If you created a myCloud before the 16th of November 2017
Your username is one of your valid email addresses. If you have forgotten what email you have used to register we suggest you try with all your valid email addresses. To do that go to myCloud.ch and click on "login" to access the dialog displayed below:
If you created a myCloud after the 16th of November 2017
Your username can be an email address or a Swisscom Login username (e.g. 'meier1976'). To recover your username fill in the field displayed in the screenshot below on this link.
For more information about Passeport and Swisscom Login please follow this link.