A loan from a tax deferred account such as a 401(k) is not taxable so there are no taxes withheld.
If you defaulted on the loan or left the company without paying back the loan, then the distribution is taxable income and you will receive a Form 1099-R for the distribution.
To enter, edit or delete a form 1099-R -
Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Scroll down to Retirement Plans and Social Security
On IRA, 401(k), Pension Plan Withdrawals (1099-R), click the start or update button
Or enter 1099-r in the Search box located in the upper right of the program screen. Click on Jump to 1099-R
