The user ID must be easy for the person to remember, as it must be used every time MitID is used - but difficult for others to guess.
Tip: Copy the validation code into the app In some phones, you just need to tap the grey box to have the code from the SMS automatically entered - or you can see it on a banner that appears at the top for a few seconds. Finally, there are phones where you have to go to your messaging app to see the SMS and back again in the MitID app to enter the code.