Nice URLS possible in dropdown_cats?

I can’t seem to get the nice URLS working with dropdown_cats. Instead of https://www.domain.com/cat-name I can only get the default https://www.domain.com/index.php?cat=1&submit=Go

Has anyone got this working before? As a normal list everything works fine but we have around 60 categories so have to do the drop down option. Could this be fixed using mod_rewrite?

Any advice appreciated – thanks!