An Irule for Client Ssl Profile that Allows Unassigned TLS Extension Values (17516)

If you're trying to reflect the client's MSISDN extension to the server, you definitely want to use the SERVERSSL_CLIENTHELLO_SEND event. It does not require forward proxy to use this event.

when SERVERSSL_CLIENTHELLO_SEND {
    set msisdn "foobar"
    set bin [binary format S1S1S1S1ca* 17516 [expr [string length ${msisdn}] + 5] [expr [string length ${msisdn}] + 3] 0 [string length ${msisdn}] ${msisdn}]   
    SSL::extensions insert $bin
}