If you are ready to upload content from Unity and "have not" created your avatar account yet you can do so within Unity here if it is not installed already.
If you have already made a creator account, you do not need to make one, you can just sign in with our editor pack you installed in Unity.
Once it is installed, to make your Creator account, click on the 'Sinespace' button on the Unity top bar then 'Sign In'.
In the Inspector, you will see the Sinespace Login Information, along with a button 'Create an account'. Click on this button.
It will take you to the Creator signup page. Here you can choose a default avatar and enter a username, your email address and password. Once you have clicked on the 'Join now' button, it will take you to the viewer download page. Once here, click on the 'Become a Creator'. Fill in all the information under the 'Join our creator program' section. Because you have already made an account, it will auto-fill your email address. Once this is filled in, click on the Join Creator Program button, and you're now a Creator!
You can go back to the Sign in option in Unity and use your username and password to log in. This will generate a unique API key which enables uploading to Sinespace.