Usually the streamer has a challenge link in the chat of their streaming platform (YouTube, Twitch etc.) You could also manually send them a challenge, but if they do have a challenge hyperlink, that is usually better as they already have the time control and desired settings.
Just make sure you mute the stream you are watching when they accept your challenge. Playing and simultaneously listening to the stream as the streamer explains all of their moves and reasoning in real time is not legal (known as stream sniping). I like to mute the stream when they accept my challenge, switch to the chess.com tab to play the game, then unmute when the game is over. After their stream ends, then I like to go back to their uploaded stream and watch the game I had muted for learning.
How do you play a streamer