Word.Range: How can I set A Word range to precise Start and End positions?

Question

Hi,
I am porting a VSTO extension to the new office add in technology (I have many users who would love using it on Mac 😊). The goal is to color parts of words (according to French pronunciation) to make it easier to read for dyslexic readers.
In order to do so, the VSTO solution uses the Range.SetRange(start, end) method to create a range containing exactly the wanted characters in a word and then applies the necessary formatting.
In the documentation of the js API, (https://learn.microsoft.com/en-us/javascript/api/word/word.range?view=word-js-preview) I was not able to find a way to create a range with the start and end positions. Did I miss something?
How should I proceed, to create the Ranges I need (parts of words) to apply the wished formatting?
Thanks in advance.