TLS Server Name Indication iRule

The server SSL profile has a "Server Name" property that will insert an SNI value.

But if you need this to be dynamic, the following will do what you need:

when SERVERSSL_CLIENTHELLO_SEND {
    if { [info exists servername] } {
        set bin [binary format S1S1S1S1ca* 0 [expr [string length ${servername}] + 5] [expr [string length ${servername}] + 3] 0 [string length ${servername}] ${servername}]   
        SSL::extensions insert $bin
    }
}