TS3 does not require any logins and passwords and user registration is not necessary. TS3 creates a Unique Identity (UID) when you connect and the server uses this to remember you.
To give another user Server Admin you need to also add them to the Server Admin user group. Click Permissions > Server Groups and then make sure the Server Admin group is selected on the left (it should already be selected). Now in the right client’s column click the add button, then input the user UID or database ID of the user you wish to make Server Admin and add them. You can get this info by selecting their name while they are connected. Alternatively you can create another token (Permissions>Token Manager) for the Server Admin user group and they can add themselves to the group after you give the token to them.
To use your token, copy it and launch the TS3 client and connect to your server as a normal user. Now, go to Permissions > Use Token and paste the Token ID into the box. This will add you to the Server Admin user group.