Here is what I got from the link you shared:
"Replace formsite_field_ID with the id of your Hidden Field item. If the Hidden Field item is the first item in the form, the ID will be RESULT_TextField-0. You can view the form and either view the page source or use the browser’s developer tools to find the item’s ID value."
So let's start by making sure that the id of the hidden field exists and is 'RESULT_TextField-0'.
To do that, you need to inspect the page. Select an element close to the hidden element since you cannot see the hidden element. And then look for the id of the hidden element. I have included a small demonstration on how to do it on a different form.