It should go into the 1099-R section on the Wages&Income page, in the Retirment income section.... just like any other 1099-R.
BUT
Once you start in there, you must sub-select what flavor of 1099-R it is...look carefully, as yours is likely a CSA-1099-R, which has different boxes to enter form a standard 1099-R.
IF you have a TSP account.....distributions from those go on a standard-plain old 1099-R.