hi
if your key performs the keyless entry and start functions then you are ok. Best and most haslefree way to do it is order through your local lexus dealer - i have been quoted £180 to supply and program the key.
dont be tempted into buying a second hand key and programming it - the "smart" start side of things can only be done by the dealer.
words of advise - if you only have one key - its better, cheaper and advisable to get a replacement through a lexus dealer - if you lose both keys you will need to get a replacement ECU to get new keys which costs thousands.