Try going to the search box and type in SSA-1099 or Social Security Benefits. That will give you a Jump To link to take you directly to it.
Enter a SSA-1099, SSA-1099-SM or RRB-1099 under
Federal Taxes on the left side or top
Wages and Income
Then scroll down to Retirement Plans and Social Security
Social Security (SSA-1099. RRB-1099) - click the Start or Revisit button
You enter the withholding in box 6 on the same screen, right under box 5. The withholding shows up on 1040 line 16 with any other withholding from W2s and 1099R.