How do I use my airplay on my iPhone 8

Unless you have an Apple TV connected to that TV, it will not work.

Roku does not to my knowledge support AirPlay in any form.

You may be able to "Cast" from supported apps such as the Youtube App but that is all.

Cast YouTube TV - iPhone & iPad - Chromecast Help

I don't believe you can use AirPlay with a ROKU, you'll need to purchase an Apple TV.

you should be posting this question in Roku's support discussions. This is discussion is for appletv users.

https://forums.roku.com/

Jason