How to add words with diacritics to dictionary?

Example: how do I add the word akṣạra (dots under s and second a) to my dictionary? It’s a transliterated word from Sanskrit. I want to use many more like this one.
I couldn’t find a suitable language for this under Settings/Languages.

Add it like any other word - type it, click on it when underlined in red, and then select “add to dictionary”.