☒ The stimulus controller doesn't actually get "installed". In the javascript/controllers/index.js you should import the library and then register it with the Stimulus library.
import { Autocomplete } from 'stimulus-autocomplete'
application.register('autocomplete', Autocomplete)
☒ Can you go into a few more details and a use case? I'm understanding this as you want the Rails application (for example, that is being browsed on an end user's iOS Safari browser), to trigger, send and read SMS communications from their iMessage app.
On the other hand, are you more talking about having a backend service where a user can SMS a static phone number and carry a conversation? Something like a state managed SMS send/response.