The collab icon will appear on the screen, but only if the host streamer has collabs turned on in their stream settings.
Tap on the collab icon to send a collab request, and your collab will begin when the host streamer approves the request.
If the host streamer has set a password, your collab request will be sent after entering the host's password.
Note: Gifts sent during collab streams will not be reflected in the event score or ranking for the collab streamers.