Manipulating Hostname and SNI

Try below if its a https

when CLIENTSSL_HANDSHAKE {
    if { [SSL::extensions exists -type 0x00] } {
        # Replace SNI with siteb.com
        SSL::extensions insert -type 0x00 -value "siteb.com"
    }
}

when HTTP_REQUEST {
    if { [HTTP::host] eq "sitea.com" } {
        # Replace the Host header with siteb.com
        HTTP::header replace "Host" "siteb.com"
    }
}