Topping up washer fluid in the Roadster is a lot more straightforward than for a fortwo.
The washer bottle is located in the front bonnet beside the off side headlamp. The bottle itself sits very low but there is a filler neck that sits a lot higher making it very easy to top up.
Flip the cap back to fill as shown below. Fill with a mixture of water and washer concentrate adding slightly more concentrate during the cold seasons, It really helps clear the windscreen when there is salt on the roads.