Option 1: Model the repayment as an income stream
This approach assumes that you have already taken a loan from your 401k. This indicates that your current 401k balance has already been reduced by the amount of the loan.
This approach also assumes that you know the duration of your loan repayment. If that is the case, you can model an additional contribution stream from your income to your 401k to simulate repayment.
Go to My Plan > Savings & Assets
Scroll to "Do you or will you save more to your accounts?"
Select your 401k as the destination account
Enter your monthly payment amount
Enter the start age and end age (based on "You," the primary user in your plan)
Select the reason "Paid off a debt"
Option 2: Model the repayment as an expense
This approach assumes that your current 401k balance reflects the total after you repay the loan.
This approach also assumes that you know the duration of your loan repayment. If that is the case, model a debt with the description of loan repayment for the duration of your loan.
Go to My Plan > Debts
Look for "What are your current, non-mortgage debts?"
Enter "401k Loan" as your account description
Select "Other" as the type of debt
Enter your loan amount
Enter the interest rate, likely 0%
Enter your monthly payment
Monthly payments will continue until your loan is paid in full.