Host Header is a request HTTP header that CDN servers use when pulling the content from an origin server. Normally the Host header should be the same as your Origin Source.
To manage the option go to Advanced Settings, find the Host Header option and choose your CNAME or write Custom Value.
Check if the header is right
Use the following command in the terminal:
curl -I -k -H "Host:cdn.site.com" https://126.96.36.199
cdn.site.com — the host header that you specified in the Host Header option
https://site.ru — your Origin Source or origin IP
If you get the 400 Bad Request or 403 Forbidden response, your server cannot process the header you specified. You need to specify another header.