After-tax savings accounts can be entered on My Plan > Accounts and Assets.

Step 1: Open the Savings section

Step 2: At the bottom of your listed accounts, press "Add an account +"

Step 3: Select that this is an Investments/Savings/Checking account

Step 4: Choose if you would like to link the account or manually enter the account info (we will be choosing "Manual Entry" for the purposes of this demonstration)

Step 5: Enter a descriptive account name

Step 6: Enter the current balance

Step 7: Select Ordinary Income as the tax treatment

Step 8: Enter the Optimistic and Pessimistic return rates. If the account holds cash, assumption return rates should be close to 0%

Step 9: Determine if you want to add contributions to the account. If yes, head over to Money Flows.