I struggle with that as well. I use Paycheck Deduction Transaction Generator to break out all my deductions, which include a 403b contribution.
The proper way to track this would be to show the transfer out of the paycheck and transfer into the Schwab account. Then for each investment (eg. Vanguard 2050) show the transfer out of the Schwab account and the transfer into that particular investment. Then each investment would have a monthly transaction showing the “interest” which would amount to the change in the balance of the investment.
I don’t get all those transactions for my 403b, and I haven’t gotten into the weeds on manually adding all that info, so I treat my 403b deduction as an expense, and then I just let the 403b balance, which usually increases by at least that much, be reflected in my Net Worth. Would love to hear how others are handling this!