Go to tether.to/settings/apikeys to create a new API key to pair the app with your Tether account.
API key allows you to give to the app only the permissions you want.
For example if you want to read transactions but not create new ones from the app you only have to assign the right permissions.
When the API key has been created, a QR code will be shown in your browser. Now, using your app and your phone’s camera you can scan this QR code to import the API key and log in. After this initial setup, logging into the mobile app is performed via a passcode.