You need to add or remove the text from the list:
Assuming the element is called tmp3, and the state is called selected you set the value to tmp3's selected :minus item youritemvalue to remove the text, and tmp3's selected :plus item youritemvalue to add the text.
One action is triggered when the checkbox is unchecked, the other when the checkbox is checked.
my example is in a repeating group but you can use watever text you prefer (eg. parent group text, or group x text, or any arbitrary text).
You have your text somewhere, use that.
The logic is that you set the value to:
your element’s value
plus item
any text you want