There is no explicit way to just get prefix matches for an autosuggest.
However, it should be possible with the search endpoint to get the behavior you’re looking for. You could try either iterating through the results and looking for the first prefix match to use as the type ahead “hint”, or optionally, you could sort the results client side so that prefix matches are ordered before non-prefix matches.
I took the latter approach, using an engine of node modules, and was able to get reasonable results:
Hope that helps!