1. To prepare dressing, combine first 4 ingredients in a large bowl; stir with a whisk.
2. To prepare salad, place potato and 1/2 teaspoon salt in a medium saucepan. Cover with water to 2 inches above potato; bring to a boil. Reduce heat, and simmer 8 minutes or until tender; drain.
3. Add potato to dressing in bowl, tossing gently to coat; let stand 15 minutes. Stir in cucumber and remaining ingredients; toss well. Cover and chill.