You don’t have to. There’s a standard order of precedence if you do not designate a beneficiary. In descending order they are: your widow or widower; if none, your child or children equally, and descendants of deceased children by representation; if none, your parents equally or the surviving parent; if none, the appointed executor or administrator of your estate; if none, your next of kin who is entitled to your estate under the laws of the state in which you resided at the time of your death.
That order of precedence also applies if you do designate one or more beneficiaries and none of them are alive at the time of your death.
Note: If you have not made a designation since before 1996 and do not want the order of precedence to apply, you should file a new designation form. Before that time, the forms were filed at employing agencies and while agencies were told to forward all forms to the TSP at that time, some did not and still submit them to the TSP on the death of an investor. The TSP no longer honors those forms; it treats those investors as having made no designation.