Place sliced cucumbers and radishes in a colander set over a bowl. Sprinkle with salt and toss gently. Let sit for 10 minutes to draw out excess moisture.
Rinse the vegetables quickly under cold water and pat dry with a clean towel.
In a large bowl, whisk together apple cider vinegar, olive oil, black pepper, and sugar (if using).
Add the cucumbers, radishes, dill, and red onion (if using). Toss well to coat.
Let the salad rest for 5 minutes at room temperature to allow flavors to meld. Serve cold or at room temperature.