You cannot have a text index in Prime!
But if you can't, but really want to, then you can!
Your text got cut off, Valery. "it is a ..."?
Do you mean subscript rather than index?
I've been playing around with various fonts trying to find the one that looks best (for values of "best") when using Unicode subscript and superscript characters.
It surprises me, given the remarkable number of character sets that Unicode encompasses, that nobody thought to have a coherent, complete, contiguous set of subscript and superscript characters. The Latin sub/sup set is missing some letters and the ones that are there are seemingly put where there were holes available! (I'm sure they weren't but it gives that impression. Additionally, as shown in the screenshot, they aren't all guaranteed to have the same size or alignment.