https://library.adaptavist.com/entity/update-the-value-of-a-custom-field-using-a-listener does most of what you need.
You'll need to remove the "is this a subtask" check, and reverse the intent so that it updates the parent
If you hover the mouse cursor over the red X's it will give you helpful error messages.
To get you started, I think line 18 will give you something like "no such property". This is because the call you want is a method, not a property, and you've got the name wrong! Check the case of the letters in the call, and add () to the end of it to force it to look for the method.
Line 24 is probably not going to work either, as "subtask" will now hold a list of sub-tasks, not a single one. You'll need to iterate over that list.
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