I have a bot command that needs users to sign in first in order to get the answer. For example, if user types in list my saved movies, the bot will send a sign in card with a sign in button. After the user signs in, the bot should give user the answer for the original command, but currently I don't know how yet. Do you have any idea how can I achieve this?
The Bot Framework includes the ability to save different kinds of state, most particularly "user" state, which is tied to the user themself, and conversation state which is tied to just a particular conversation. One of these, in particular conversation state, could be useful - you could save the command into conversation state, do the signin, and then check from there what command triggered the signin and therefore what to do next. I haven't done a lot of work with signin, but I think that should work. More info on this is available at save user and conversation data.
Just to confirm, I see you've tagged the question with MSTeams, so I assume the command you're offering the user requires access to, say, the Microsoft Graph or similar? I'm asking because you don't necessarily need to authenticate the user per se, if they're just accessing data in your own database, as an example - they're authenticated via the Teams client already - you'd want signin, I guess, in order to get a token so that you can call something else on behalf of the user (e.g. OneDrive)?
Hope that helps