See the screenshot above. Everything seems to be setup properly to fill in the correct form name but it’s not working. Here is also a screenshot of the form in the Designer:
Do you mean that literally, with brackets and parens?
It looks like you’ve specified your variable names correctly, e.g. {{ formName }}, so that would be a glitch and you’d need to contact support.
It could be that you’re on a free plan and perhaps that feature doesn’t really work correctly on the free plan?