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I'm afraid the Rich textbox doesn't have an option for your users to enter their text as superscript nor subscript.
However, the TinyMCE widget might do the job https://widgets.jotform.com/widget/tinymce
This widget will allow your users to format superscripts and subscripts, like so:
Here's a guide on How-to-Add-a-Widget-to-your-Form
I understand you want something "simple" but after checking different widgets, this is the only one that offers the easiest way to implement super/subscript.
Hope that helped somehow :)
Looks perfect! Thanks for the quick reply, I'm excited to give it a try.