Combine cucumbers and salt in a large bowl; cover and chill 1 1/2 hours. Drain; rinse cucumbers under cold water. Drain; return cucumbers to bowl. Add onion to bowl.
Combine sugar and remaining ingredients in a medium saucepan; bring to a simmer over medium heat, stirring until sugar dissolves. Pour hot vinegar mixture over cucumber mixture; let stand at room temperature 1 hour. Cover and refrigerate 24 hours. Store in an airtight container in refrigerator up to 2 weeks.