How to use etags for caching?

问题

Can anyone tell me how to pass etag correctly for this url?

https://www.googleapis.com/youtube/v3/playlistItems?part=snippet,id&playlistId=UUwpy_3CqtfwM7tg2JgQQQyA&maxResults=50&key={API_KEY}.

I was passing like this, but didn't work.

https://www.googleapis.com/youtube/v3/playlistItems?part=snippet,id&playlistId=UUwpy_3CqtfwM7tg2JgQQQyA&maxResults=50&key={API_KEY}&If-None-Match="p4VTdlkQv3HQeTEaXgvLePAydmU/vKZr9jiSytaO7UdTbhKbIrwbX20"

来源：https://stackoverflow.com/questions/59587979/how-to-use-etags-for-caching