Linking the accounts you create in the Invest section of Portfolio123 is very easy.
When you create a new invest account you will be asked if you wish to link the account to your brokerage account. You can also link an account later by accessing the account properties.
You will need to agree to IB terms and provide a picture ID. The approval process takes about 2-3 business days.
Currently only Interactive Broker accounts are supported. The ability to link to other brokers is underway.