Just string replace with {width} and the {height} with the width and height you want.
Fetch https://api.twitch.tv/helix/streams?user_login=cohhcarnage
Response is
“thumbnail_url”: “https://static-cdn.jtvnw.net/previews-ttv/live_user_cohhcarnage-{width}x{height}.jpg”
I want a 400x200 so I take thumbnail_url and replace {width} with 400 and {height} by 200
Results in:
https://static-cdn.jtvnw.net/previews-ttv/live_user_cohhcarnage-400x200.jpg
Done