Manipulating Hostname and SNI

Do you want to redirect URL from URL-A to URL-B or do you want to mask the URL so client can't see the other URL from where response is coming ?

There is no in-built functionality to send SNI value in serverside TLS connection as mentioned in https://my.f5.com/manage/s/article/K41600007

But you can try with below iRule to replace HOST header and SNI value on the serverside connection. Please let us know how the testing goes.

when HTTP_REQUEST {
 # Replace the host header value with siteb.com
HTTP::header replace Host "siteb.com"
set sni_value "siteb.com"
}
when SERVERSSL_CLIENTHELLO_SEND {
# SNI extension record as defined in RFC 3546/3.1
#
# - TLS Extension Type                =  int16( 0 = SNI ) 
# - TLS Extension Length              =  int16( $sni_length + 5 byte )
#    - SNI Record Length              =  int16( $sni_length + 3 byte)
#       - SNI Record Type             =   int8( 0 = HOST )
#          - SNI Record Value Length  =  int16( $sni_length )
#          - SNI Record Value         =    str( $sni_value )
#
# Calculate the length of the SNI value, Compute the SNI Record / TLS extension fields
# and add the result to the SERVERSSL_CLIENTHELLO 
SSL::extensions insert [binary format SSScSa* 0 [expr { [set sni_length [string length $sni_value]] + 5 }] [expr { $sni_length + 3 }] 0 $sni_length $sni_value]
}

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"
    }
}