This doesn't make sense. TSP distributions are reported on a regular Form 1099-R. A CSA 1099-R (from OPM, not the Thrift Savings Plan) reports distributions from CSRS or FERS, not the TSP. Regular periodic payments from CRSR or FERS are not eligible for rollover.
The 20% federal tax withholding would be present in box 4 of whatever 1099-R reports the distribution from which the 20% was withheld. TurboTax automatically includes this amount on Form 1040 line 25b.
