How to run rocketchat like "https://mysite.com/chat" on Debian & caddy

My caddyfile already insert like this :
mysite.com/chat {
proxy / 127.0.0.1:3000 {
header_upstream X-Forwarded-Proto {scheme}
header_upstream X-Forwarded-For {host}
header_upstream Host {host}
websocket
}
}

And What should I insert in rocketchat.service “ROOT_URL”

why everytime i changed “ROOT_URL” and restart rocketchat, the “Site URL” are not change?

help ! thanks!