I suggest to use nginx as reverse proxy and setup ssl in there, the library for setting up ssl in Ktor seems to be for testing purposes as mentioned in Ktor docs website, even if not, the docs recommend to use reverse proxy solution
It's usually easier to setup ssl in there, I have a guide in GitHub gist, it's outdated and I wouldn't recommend it, it's still can be useful.
Edit: Didn't know you were talking about Ktor client