You need to use C# to load (GET) a web URL - https://api.twitch.tv/helix/streams?user_login=lightstreak Replace lightstreak with the user you are interested in.
You need to set a Header with your Client-ID.
Then, when you load the URL, you will get a response in JSON format - this is why you need to parse JSON.
If the user is not online, you will get a response like this:
{ "data": [], "pagination": {} }
If the user is online, you will get a response like this:
{ "data": [ { "id": "28110269104", "user_id": "26490481", "game_id": "497078", "community_ids": [], "type": "live", "title": "FC5 PC First Playthrough - [ Follow @summit1g ]", "viewer_count": 25871, "started_at": "2018-03-28T19:29:14Z", "language": "en", "thumbnail_url": "https://static-cdn.jtvnw.net/previews-ttv/live_user_summit1g-{width}x{height}.jpg" } ], "pagination": { "cursor": "eyJiIjpudWxsLCJhIjp7Ik9mZnNldCI6MX19" } }
Note that type is “live”