You can add a model to your Studio account if the model follows these simple steps:
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1. She needs to log into her CMD account

2. She needs to go to Settings ⇒ Account and click on the "Add me under studio" tab

3. There, she needs to write your Studio's username or your Studio's Skype ID (the variable you use to log into your account) and click Save

Make sure you give her the correct CMD username of your Studio or your Skype ID, to avoid getting the account linked under the wrong Studio account.