Combine the sugar and water in a small saucepan and bring to a boil, stirring occasionally. Meanwhile, wash the cilantro and cucumber, then slice the cucumber into 1/4-inch rounds.
When the syrup hits a boil, remove from heat, add a pinch of salt, and cool at room temperature for 15 minutes.
Add the cilantro and cucumber to the pot, close the lid, and steep 4 hours. Overnight would be better.
Strain syrup into a sealable jar and refrigerate for up to 3 months...or until it grows funny stuff. If that happens, throw it out. I’m not saying it will happen, but you can never know for sure.