The first buyer of your Smart Roadster neglected to tick the “cruise control” box?

Never mind. You just have to buy the cruise/wiper stalk, hammer it in in place of your existing wiper only one and assuming that your car has been prewired for it (most were) you plug in the previously unused plug and off you go.

EDIT: There you don’t go, because you have to get the ECU reprogrammed by a dealer.

No extra hardware needed (unless you’ve drawn the short straw and haven’t got the wiring in which case you buy or make the extra harness).

As to why anybody would want cruise on a car singularly ill suited for long distance travel on a motorway is another matter.