As i remember it should return the 64bit steamid as an FString if the platform is Steam.
FString SteamID = GetWorld()->GetFirstPlayerController()->PlayerState->UniqueId->ToString();
/** The id used by the network to uniquely identify a player.
* NOTE: the internals of this property should *never* be exposed to the player as it's transient
* and opaque in meaning (ie it might mean date/time followed by something else).
* It is OK to use and pass around this property, though. */
UPROPERTY(replicatedUsing=OnRep_UniqueId)
FUniqueNetIdRepl UniqueId;