On iOS, you can set an accessibility value with the accessibilityValue or accessibilityAttributedValue property.
When using the semantically correct element, you usually do not need to modify the accessibilityValue. For example, a UISwitch sets the accessibilityValue to selected or not selected and a UISlider sets the accessibilityValue to the current value. If the default value is incorrect or unclear, you can override the value manually.
element.accessibilityValue = "Custom"