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.