Hi @michael.chen - thanks for posting on community. I am just adding my respond in support center here also for visibility 
Upon asking internally, it sounds like not all Identity Providers(IdP’s, i.e. Twitch) support the prompt parameter. So, you will need to look at upstream_params (Pass Parameters to Identity Providers). This allows you to send something to auth0 (eg: prompt=foo) and then auth0 will send that to the IdP (in the same parameter or a different one).
Let us know if that works for you!